RE: Roundtuit
The Pizza Box Flier was a 24" square. It could easily be cut in a circle. I'd make it larger for a .40 as the PBF was designed for a .25. Maybe 30" in diameter. It was cut from 4mm Coroplast. run a few carbon fiber tubes down the flutes to keep the wing stiff in case you fly too fast.
The "fuselage" was a 1/2"-3/4" aluminum channel from Lowe's. It was just wide enough for a standard servo to snugly fit into and be held with a zip-tie. The engine mount was 1/4" ply, or hardwood bearers-your choice. Another option is an engine mount cut from a plastic cutting board from the kitchen section of Wal-Mart. This is not a precision project.