I attempted to use foam squares and batting for the seat cushions of the chairs I'm upholstering. Bad idea. It would have taken an insane amount of batting to potentially make the chair look OK... so instead I'm re-using the old cushion in its entirety and scratching the foam idea.
This realization came to me after cutting the corners off of one of the foam cushions to match the chairs shape with my foam cutting knife. I then thought the cushion was too thick and should be cut in half... which I did... and was covered in foam chunks (don't wear a sweater that collects everything in sight when cutting foam).
Looking back on it, I think I could "make it work" (oh, how I love Tim Gunn!) but oh well. The foam cushions were pricey, so now I can return the one I didn't use.
On a related note, the foam cutting knife was OK. Better than scissors... but still not great. I've heard the best way to cut foam is with a serrated electric knife, but I didn't want to spend the money on one.
Now for the fate of the cushion I butchered... I'm thinking it will be made into a dog or cat bed. Probably a cat one... the dog still likes to eat his.