Mindwarp is right with the foam installation. Mike and I wrote at the same time, so you get twice the info.

I guess its optional but I would either screw the light ply to some existing wood. Usually a fuse former at the back of the space for the tank. Put foam between that and the tank too.

Its not set in stone but I think access to the fuel tank from the rear is a needful thing.
Otherwise you end up removing the engine to access the rubber seal of the tank to remove its insides.
Your fuel pick up clunck can fall off..lines crack.

The fuel clunk attches to the metal or platic tube inserted through the rubber seal in the front. Pressure nipple beside You want to make sure that pick up line inside the tank is clear of he rear of the tank. Close but not touching. Hold the plane or the tank depending on where you are and see that the clunk moves freely form top to bottom, side to side without biding on the back of the tank. Each day before I fly I flip my planes around to listen for a free moving clunk.

Here is a pic of my Avistar instructions. They explain better. This is typical for all planes....several optional ways of doing these lines are in use. This is basic.

By the way the "Caution" you cant read say's not to bend the pressure tube up so much it touches the tank or fuel flow might be interrupted...do not overtighten the stopper plates or damage to the tanks might occur.

Good luck
Michael