The original design used1/4" balsa for the engine box/wing tube//landing gear box. This was surrounded by 3/8" square balsa. I substituted 1/4" lightply for sides and provided lightening holes. The original floor was 1/4" lightply, I used 1/8" lightply except under the gear where 1/8" aircraft ply was used. I cut the pieces to interlock much like a currentARF, this takes the bows out of the lightply.