We need to know what kind of performance you are expecting from your motors. That way we know how strong the material and the attachment process needs to be.

1/8" balsa will handle D and E power. 3/16" E and F, 1/4" balsa with a little epoxy coating can handle a full G160 surface mounted!!!!! My old buddy Jim had an FSI Black Brant that we could not destroy with his EX motors, and they were great motors.

Beyond that, 1/8" aircraft plywood will handle up through H and I and 3/16" will handle J and possibly more if shaped correctly and attached through the wall. It will take them a long time to rot, though, especially if coated with epoxy.

As a reminder

A 2.5 n/s
B 5 n/s
C 10 n/s
D 20 n/s
E 40 n/s
F 80 n/s
G 160 n/s
H 320 n/s
I 640 n/s
J 1280 n/s

__________________
I love sanding.