I made my flap afterwards but I strongly recommend that you build it in from the beginning. The flap was made 75 mm x 435 mm (approx. 3" x17"). I placed the Hitec HS84BB servo about 60 mm (2,4") in front of the flap and mounted the servo with servo tape on the 55 x 60 mm (2,2" x 2,4") hatch.

I made the flap from two sheets of 0,040" plywood laminated together with epoxy. With the 90-degree flap the model almost stopped in the air with the nose pointing almost strait up! I gradually lowered the extension and now I have 35mm (1.38"), that’s about 40 degrees.
I have mixed in a down compensation on the elevator of 6mm (0.25") and this makes lowering the flaps uneventful.

I have three pictures but they where too large to include. E-mail me privately if you want copies.

Best regards
Thomas