RS2K:

On this airplane, unless you want to strip the planking and start over, just live with it. You have sanded the skin unevenly, by the picture, and unless you do some major sanding (consider your wing) the surface lines wont match.

You can minimize the gap by sanding the matching faces and adding a sheet of balsa to the end of the hatch, or even using thin ply on both sides.

For the next one, as you frame it up leave 1/16" clearance, then after sheeting and before sanding, put a 1/32" ply end on the hatch, and a matching piece on the plane. Gives minimum gap and reinforces the edges to minimize later damage.

Picture attached is a plywood skinned hatch.

Making really narrow hatch gaps is one of the hardest things to do, building an airplane.

Bill.