After a couple of trips I've added a couple of things.
First, I added a skirt along the bottom of the hatch to shed water. It's a piece of plastic garage door seal and so far it does the trick:
Next, I added a battery box to the tongue. A friend at a metal yard gave me a piece of steel for a base and I used a marine box. My battery is too big for the back and I didn't like having to carry it separately.
I'm having some problems with the hinge leaking. It doesn't seem to be bad in rain but really leaks when I wash it. I'm probably just spraying water in a direction it wasn't designed to take. I'm not too worried about it as I'm working on a rubber seal for it and the inside is very well protected with epoxy and polyurethane.
Excuse the dust, she's been on a few trips in the desert! One was a little more "off-road" than I expected. The stuff in storage spaces got scrambled pretty badly but the structre held together well.