Use what ever you got! some say reticulated foam wich is probably better to work with but I don't have a supplier close by so I use aploshstery foam, foam I can get at a Fabric store.
I would think that Reticulated would be more Breathable, and lite due to it's more porus nature.
For me it's just not worth the hassle to order off line and deal with shipping bulky material. So I go to Hancocks and by it by the yard.