Originally Posted by Boomerang1

Cut a piece of 3 mm ply & cut the front to a point to fit into the nose of the model.

Then glue some balsa block to the top & bottom of the ply & sand it to a cone shape to wedge into the nose of the model.
This will secure it up & down & side to side.

At the rear secure it with some screws to some hardwood glued to a former or some hardwood glued to each side of the fuselage.

Hold your batteries in place on the ply with some balsa strips & zip ties. This idea worked fine on a Savex F-16.