I probably should have let the thing cure a bit longer in the mold but it looked solid enough and I just couldn't wait. I'm that way with scabs, too. And like a scab unready for the picking, it took plenty of encouragement to bust the mold loose. First some prying between the flanges, then a bit of drumming on the hull to pop free along the surface, more prying, more pounding, you get the idea. These things stick together quite a bit because of the PVA. It's weird stuff: a thin membrane that starts off applied to the mold but when you take everything apart, it ends up on as a coating on the final part that you just rinse off with water.
This is how it looked immediately out of the mold, so a lot of boogers and some flash to clean up, but the part seems pretty sound!
I ran it out to the garage to confirm fitment with the bottom, and after roughing out the edges a bit they look to be mating pretty well.
Next step will be to double up a few more layers on the top part from the inside where it looks a little thin, then it's on to fitting up the petcocks and gas cap.