I wrapped the battery pack in two layers 1/4 inch foam. I then wedged it all the way up against the firewall, underneath the fuel floor. With the two layers, it fits nice and snug and won't move during flight.

For the fuel tank, I also wrapped in 1/4 inch foam, that should take care of side to side movement. As far as front and back, I pushed it against the firewall and then secured the back of it with excess balsa, created sort of wall brace behind it. The balsa is strong enough to hold it and is easy enough to break if you ever need to remove the tank. I also CA'd a piece of balsa over it (CA the sides to the fuselage), however, keep enough clear that when you remove the hatch, you still have access to it. In the end, make sure it is secure, you don't want that sliding around.

Good luck, it's a great plane