I pretty much do as KadetBuilder does, but I'll do anything to not add additional ballast.

I have (4*60) cut out the covering over the bottom lightening hole two formers back from the servos, installed a piece of lite ply and velcro'ed two battery packs there. Perfect balance.

I think you have to be open to cutting covering. If you have a match to the covering material just patch it afterwords, otherwise any color will work fine and you can make a pattern out of it if you want.

Short-coupled planes that are tail heavy don't give you the same options, but nose heavy ones invite a rearward battery placement.

Good luck!