Unless you got the foam pre-cut for your specific cell (and that's rarely the way you get it - it's usually just in big blocks), what you also want to do is pull out the old foam first then use the old pieces as a template for cutting the new stuff. It makes the whole installation a lot neater than trying to figure out a pattern yourself.
Don't worry about compressing the foam to get it in and out. I recently replaced an 8" x 8" x 16" block thru the 3" opening of a "stock car" cell with minimal problems.