You want the stretch of the fleece from front to back for maximum comfort. Side to side will pull it too tight front to back while weighted, but some on here have indicated in the past they like it that way. I tried a piece of foam last fall, but found that it was more comfortable without it for me. You do have to pull and spread out the fleece more before you sit or lean without the foam. You can try all the commercial options available, and even those coming soon, and I can guarantee you that none can come close to the fleece!
Since you mentioned sewing already, you should consider sewing a channel and making the ends like a mushroom vs the folder over method. There is another thread about doing that on the site, but can't search right now on my phone. It really cleans up those ends and cannot possibly pull out. I saw the folded ends move on mine over time and switched.