How To Build A Teardrop Trailer/The Hatch
The Hatch 
For some reason, the construction of the hatch evokes fear in a lot of new builders. I have read builders delaying the hatch until the very end of their build. There is a point to their fear. The hatch is constructed by bending luan over a wooden frame, and one problem with that is a problem called "bow-back".
Bow-back is where the force of the bent luan pulls the whole hatch out of shape and causes a gap at the bottom. This is due to builders attempting to build the hatch with contoured ribs, so that the hatch is thin (the ribs being 1.5" - 2" wide). If the ribs can not withstand the force of the luan bent around them, the ribs bend. Many builders make a second hatch after the first one failed.
For our simple teardrop, we will avoid the entire bow-back issue by keeping the back edge of the rib straight. With a straight edge there is no possibility of bow back (via simple engineering mechanics). It behaves like a form of triangulation. The downside is that, as you can see, the ribs are rather bulky. The upside is that they are so strong you only need two on each end.
Making a Cardboard Hatch Rib Template 
First you need to make a cardboard pattern for the end ribs. The shape is too complex to try to measure out, it is easier to try, cut, and fit the pattern (see image for reference):
- Get a piece of cardboard.
- Trace the curve of the sidewall for the length of the opening onto the cardboard and cut it out.
- Hold the cardboard up to the sidewall and mark and trim to the angles of the floor and the angle of the spar at the top of opening for the basic rib length.
- Cut a straight back on the rib template allowing for a hatch spar at the bottom and at the top (about 2" from the curve on the angled lines).
- The end result should look like this.
Check the template on both sides of the hatch opening to make sure all the angles and edges fit well. It doesn't have to be perfect. You should be able to pivot it from the top outside corner of the template (from where the hinge will be on the body) without any interference on the bottom with the floor. The bottom of the rib should come to rest on the floor with curve remaining flush with the sidewall all the way to the top. With the rib resting on the floor and the template curve flush with the curve of the sidewall, the top of the template should only have about a 1/8" gap between it and the body spar at the top. If the template works for one side but not the other, then simply make a second template for the other side.
The Luan will overlap the hatch frame on both sides and the bottom. This overlap will be a surface for weather stripping and will contact the edges of the sidewalls and the end of the wood bed. The luan on the top will be cut flush with the hatch frame.
You can then use the template(s) to cut out 2 ribs from the same 3/4" plywood material as the sidewalls. The next step is to attach four 2"x2" spars horizontally between the ribs (2 on the top and bottom and 2 evenly spaced in the middle) to form the complete hatch frame. Spar length should be chosen so that there is about 3/8" clearance between the outside of the end ribs and the inside of the teardrop sidewalls. Screw and polyurethane glue these items together.
With the glue still wet, position the frame inside the teardrop hatch opening and use long wood screws to screw through the ribs from the inside out into the teardrop sidewalls to hold the frame square in place while the glue dries (Be careful not to screw all the way through the outside teardrop sidewall!). The frame should be positioned so there is equal distance on both sides between the hatch end ribs and sidewalls; and the top of the hatch frame is in the position required for fastening the hatch hinge (i.e. the gap is about 1/8" at top).
After the hatch frame dries overnight, you can cut a piece of luan so that extends from the bottom of the wooden bed (overlapped) to the top of the hatch frame flush with the top edge of the hatch frame spar (and there should be about 1/8" inch gap between the top spar of the hatch frame and the teardrop body spar which is at the top of the hatch opening.). As was done with the luan roofing allow the luan to completely overlap the sidewalls to be trimmed flush later.
With the hatch frame still held in place by temporary screws, you can then glue and screw the luan skin over the back of the hatch frame while it is positioned in the teardrop hatch opening on the trailer.
The next step is to use the router again with the laminate trim bit to trim the overlapped luan of the hatch to the teardrop exterior sidewalls. The hatch construction is complete and can be freed from its temporary screws after the glue dries.