DIY Window Seat Bench Cushion with Fitted Box-Corner Cover

Now that the crafts stash has (finally) been semi-unpacked at the new house and the sewing machine liberated from its moving box prison, the long awaited home decor DIYs, makeovers, and upcycles have begun.  Our master bedroom has a window seat, plainly finished in the same paint as the walls. Blah! Not comfy at all! Time to make pampered pooch Humphrey his long overdue custom window seat cushion for basking in comfort!

Fabric selection was easy for once (and "free") since I had a large piece of the same velvety grey upholstery fabric used in our custom pet basket bed cushion inserts and easy envelope pillows (the velvet makes these irresistibly comfy) big enough to cover a window seat cushion, but would need a large piece of foam for the cushion body itself.  The seat isn't going to be a high-use area (although sunbeam-loving window-watcher Humphrey may have other ideas) so it didn't need a particularly high-performance/resiliency foam insert. As a lesson learned when shopping for the pets' beds, I waited for a great sale on a good quality foam mattress instead of buying custom cut-to-measure foam, and then cut it to size. In the absence of special tools, foam cuts very well with just a simple bread knife or an electric kitchen knife, if you have one.

• Measure the bench seat area carefully.
• Lay foam flat and measure/mark the cutting dimension(s).
• Cut foam.
• Dry fit the foam in position on the seat to ensure it fits correctly in position.

I wanted the finished cushion to be as clean and simple as possible, with minimal visible seams and no piping or tufting. They just gather fur at our place! Rather than creating a true box cushion cover using top, bottom, and side panels (see detailed post and diagrams here), I opted to simplify things by boxing the corners on a wrap-around cover.  Only the front of the cushion is visible when in place on the bench, other than one little bit at the left corner (exposed overhang), so the visible side seams weren't an issue. This method is much quicker and easier to cut and sew, especially since I was working with slippery velvet that liked to move and refused to hold when ironed. Grr.

The cushion cover was made from a large single piece of fabric, with an overlapping rear closure through which the cushion could be inserted and removed if the cover needed to be cleaned. I opted for Velcro-style hook-and-loop in lieu of fitting a zipper, using some of my large rolls from AliExpress. It was made in the same way as our dog bed covers, as detailed on partner blog Dalmatian DIY (see details posts and diagrams here), with a few adaptations to combine the single piece construction with the rear overlap and Velcro. This required a little extra care on fit-checks and boxing up the rear corners where the sides meet the overlapping closure panels, but not too difficult with a little patience. 

Tip: I like to do a check fit at each stage as I work, just in case! Mistakes and rework suck, but better when minimised if ever needed!

• Measure and calculate the fabric requirements to fully enclose the cushion on all sides. 
• Add additional allowances for the seams, finishing the edges on the rear panels, and overlap of the rear closure. I included an extra wide doubled-over seam for the outside of the rear flap. I know...no one will ever see it or know it's there, but it echoes the styling used on the same velvet cushions and basket beds. It also allowed me to first attach the outside hook and loop to what then became the underside of the fold, concealing the back side of the stitching.
• Finish rear closure edges.
• Double check your measurements, confirm placement for hook and loop, then sew hook and loop into position (see alternative assembly note below), ensuring that the cushion cover will fit tightly when enclosed and fastened. Tip: You can run the Velcro so that it encroaches on the area to be sewn and boxed (no need to waste more by going all the way to the edge), or stop shorter for less Velcro use and easier sewing/boxing. There will still be plenty of hold with a small gap at each end. The difference can also be sewn, if you wish.
• Close the rear as it would be in a finished cover, double check your measurements, and then carefully position the cover right-side-in. Ensure that the fabric is folded PRECISELY where you want the Velcro closure to be on the rear. The fold lines will be the middle of the front/back in the finished cover once sewn and boxed.  Pin to secure. 
• Sew the sides together along the seam allowances. 
• Trim excess, if/as needed.
• Starting with one corner, position by pulling the fabric sideways from the seam so that the seam line is centred evenly from the point, forming a triangle. Ensure that it is flat and even. If the fabric allows, iron to ensure that the seam is pressed flat and the square holds position. Measure across and mark a straight line a the point where it is the depth you want for the boxed corner (in this case, the cushion thickness). Sew across the marked line. Trim loose threads and excess.
• Repeat for the other corners, taking care to ensure that the shared seams are flattened to fall in the same direction for a smoother line in the finished cover. Tip: Extra care is required when boxing the corners with your overlapping rear panels. Ensure everything is fitted smoothly in the as-closed position before sewing. Double check that nothing has been accidentally shifted or bunched before trimming.
• Open the Velcro closure and reverse the cover right-side-out.
• Squeeze the foam and wrangle it into the cover through the opening, taking extra care to ensure the corners are tightly in position. Close up the Velcro, place it into position on your seat, and enjoy!

Alternative assembly: If you aren't confident with the precision needed on measurements/fit for finishing the rear first, you can half sew the sides from the front fold-over, box the front corners, and do a fit-check to confirm your measurements before doing final cuts, edges, and Velcro on the rear closure. make sure you line your stitch path up when you finish closing out the sides. I've tried all both methods, and the results look the same either way, so go with what feels right for you!  You can also make the complete envelope and then sew the Velcro after, if the cushion is large enough. to freely access the flaps.

Dalmatian DIY's detailed box cushion pet bed cover instructions are available here, including step-by-step instructions, pictures, and diagrams for different styles of covers. You can also check out our envelope-style cushion covers here if you'd prefer to skip zippers/Velcro all together and just use an overlapping bottom closure (it's super simple) like our pet basket bed cushions.