I use poly ply tabs to keep the center part of most of my hatches flush th the edge of the fuselage. Set the poly ply into the hatch tack it with CA then aeropoxy them into place. Measure and cut you slot in the fuse and adjust if necessary fo ra good fit. Round the edges on the tab so it slips in and out of th slot easily. If the hatch tries to flatten out you can always add a small ply former.

DR