I started on the front hatch here.

My style is to build a frame inside the existing fuselage, lift it out, and glue it onto a blank. Later, I will trim the blank to the fuselage and then cut the front of the blank at a 45 degree and glue that piece onto the fuselage. This gives the firewall a little extra strength and the 45 degree angle should hold the front of the hatch down. This method is from the Sweet Stick plans. On that, the rear is held in place by the wing. I was thinking on this one, I'd add a tab that would engage the front wing hold down pin.