I agree with the responses you have already gotten. My personal foam of choice is Touch N Foam (black landscape version). I always put a thin layer of silicone down first, gives better bite for the foam to stick too. You can also silicone egg crate onto the back/side walls and let it dry before you foam it to give extra strength.
I also find that I can use a can for a day or two by just cutting back the tip a little, but plan on using it ASAP. Thin multiple layers are better then trying to do in shot. I also usually silicone cork and driftwood in place first when possible, allow to dry before foaming.
Just my 2 cents....