A cushion like the one in the video is often seen in a caravan, camper or boat. Often the foam is glued to a wooden board. I’ll show you four ways how you could make such a cushion. Namely:
Very simple : a small seam on the four corners
Slightly more difficult : to stitch two sides
A little more difficult : to stitch two sides, a front and back
The most difficult or actually the most laborious : two sides, a front and back with a welt.
Watch the video below and you’ll know everything you need to know about making a cushion on a plywood shelf. And I know, there are many other models with, for example, an extra raised knee area and such. But that’s for another time. The video has chapters. Check the box under the video.