So back to the top skin mold, attempt 2. I did more reading and learned that for taking panel molds like this you want a flange around the outside. I put this together with foam board, packing tape, hot glue, and plastilina wax-based clay (because I couldn't find any soft wax to seal the gaps).
More carefully applied a new "tooling gelcoat" designed for this and the top layers of cloth.
Flipped over and started on the ribbing, re-using the flanges, all part of this technique as recommended.
Separated the parts. Things look good! Mostly.