First, you should put some burlap material on the springs then the foam. This should keep the seat springs from riding into the foam.
Got an old electric carving knife, they work great for foam...
Upholstery shops also have rolls of cotton batting, sometimes this works good if you are having problems with the seat cover fitting properly. If you roll it out over the foam, before installing cover it will fill in the corners nicely.
Just my opinion, and I'm sticking to it.
RT1
**
