Step 25: Alternative #6 - cushions
There are plenty of options for the cushions. In my futon I used a (non-memory) foam mattress topper along with the filling of some old pillows, as well as some Poly-fil. The end result was a 4 inch mattress, which is plenty comfortable.
My initial design used 2 foam toppers and 4 pillows, but I found that a layer of foam that thick didn't breathe very well, so I opted for pillows/Poly-Fil instead.
You could use any combination of pillow, filling, and foam topper.
If you have the money to spare, a memory foam mattress topper ($60) would be great. In that scenario, I imagine it would be best to place the memory foam onto the fabric first, followed by some non-memory foam, then the plywood. But it'd be best to experiment and see what feels best to you.
There's also actual upholstery foam, which, if you can get access to, would be great. This stuff varies in price depending on the quality and sizes, but will generally be more expensive than pillow and foam toppers. This is the stuff used to upholster sofas, boats, etc...
You could recycle some old sofa cushions as well, but just be sure they're clean.
Lastly, there's poly beads. This stuff is essentially the filling inside bean bag chairs. Although comfortable, the material is a huge insulator since it's mostly air. I've heard of people using this stuff for in-wall insulation. So keep in mind that it will likely trap your body heat very well, and could be uncomfortable in warmer climates. I also imagine noise would be a problem if you were to move around a lot when you sleep.
I welcome you to use any combination of the materials above, or anything else you can come up with. Use whatever you can afford, or whatever is most comfortable for you.
If you have the sewing skills, you can "shape" the fabric by sewing corners into it. It's not necessary, but does help with aesthetics a bit. Instead of 'domes' of cushion, you'll have rectangles.
Also, if you're feeling extra adventurous with the sewing machine, you can make the cover completely removable. Add a zipper to the fabric and you have a removable cover. Just be sure to buy extra fabric since the cover must wrap around the bottom as well.