Next step: An aluminum cover and skirt for the hatch door.
I measured, marked, and cut a sheet of .032 aluminum to wrap around the door and down a few inches past the seal.
The corners at the front (near the hinges) require a funky cut to avoid interfering with the opening of the door to almost ninety degrees.
The other two corners are pretty simple.
I fashioned a simple metal bending brake from several pieces of wood and a lot of clamps and was able to bend the aluminum into what I wanted. That part wasn't fun, but I probably saved myself fifty to a hundred bucks by doing it myself.
This is a hinge corner from the inside with the cover upside down ...
... and from the outside as it will be installed.
At this point I was planning to weld the joints with Alumalloy, but that quickly turned into an ugly mess. (I need more practice or the metal is too thin or I was using too much heat....) Instead I decided to use aluminum tape (the kind used for sealing roof gutters) and caulk. It's watertight and that's what matters.
The cover will be fastened to the door with four carriage bolts. I don't want to glue it down because I may need to access the hinges someday and glue would make that very difficult. The carriage bolts will get acorn nuts on the inside to hide and dress up the bolt threads.