I use a type of foam than is used for packing artwork it tears into 1X1 squares, or strips,any shape you need, its very light weight, if you think the foam your using is heavy then just check the CofG if it's looking like the nose is dipping a little too much the shift the reciever /battery pack slightly rearward to get it to balance.
a slightly nose heavy plane is OK and a little foam will not add too much weight but when adding any weight to any part of a plane being foam or repair always check the balance and CG.