So cute! What did you use for the cushion? Did you have a piece of foam custom cut?
Randomly, I had the foam already - they used to be backrests on my parent's bed for many years. So I just had to cut it down to size. But for Ben's cushion a few years ago I did buy foam from a fabric store. It's not cheap, either. I've also seen it occasionally sold at fed Meyer/target
During the summer camping season, that is. My advice is to get the thickest, most dense stuff you can afford. Or even buy existing cushions and recover them.