All three of the 4 star 40's that I have set up had .46FX's in them and all had the battery in the same location.

I placed them right behind the rearmost radio compartment bulkhead(the one at the trailing edge if the wing), wraped in foam rubber and snuggled in between the pushrod tubes and the fuse top ply(on top of the pushrods). The easiest way to do this is to remove the covering from the bottom of the fuse right behind the wing so you can work the pack in place with your fingers through the hole in the bottom of the plane. There is more than enough resistance to shifting fore/aft between the foam rubber and the surrounding structure. The battery packs were all 600 mah flat Futaba packs, not the square packs. I doubt you could get a square pack into that location without some cutting and hacking.