Pics? Just like any rounded chair cushion, you cut the top and bottom shaped to fit the bench and connect them with a straight strip of fabric. For the back, cut one piece to fit the back, and a slightly larger piece to allow space for stuffing or foam.

We're turning the area underneath our basement stairs into a reading nook for my daughter and are trying to determine the most affordable route to get a cushion/mattress with a washable cover for that area. The cushion needs to be 36" deep and around 68" wide. At 68" wide, we could go 6" tall. Unfortunately, 72" is too wide. I've looked at various mattress sizes but can't seem to find anything that will fit quite right. Should we order custom foam as people do for benches? Or try some other route? Thanks for your help! This is what the area looks like (still need to finish drywall):

Assuming this will be used with a table, I don’t think a back cushion is needed. If you’re worried about folks bumping their beans on that ledge, you should probably just remove it altogether. As for the seat bottom cushions, you could have three made in this configuration: one straight and two “J”-shaped to cover each corner and end. Make sure to add heavy-duty Velcro or snaps to the bottoms so they stay put!

Pictures and more info will likely get more comments and help for your issue.