Construction question

Hi Everyone. I'm hoping to get started on building my own teadrop once I get some plans worked out from an old set I downloaded from a group. Sort of a hybrid of old and new. The only hitch I'm having is that back hatch construction to make it watertight. The rest I can do. (Farmer inginuity. Okay so I can spell worth beans.) But getting that back hatch to be watertight is driving me nuts! I'm hoping to use plywood (4' X 8') and 2" X 2" lumber from the frame with 2" X 4" for the base for strength. Styrofoam to insualte it and some LED lights I want to build for interior and exterior lighting. If anyone can help I'd be greatly appreciative.
From the great white north,
Wayne
From the great white north,
Wayne