Ah then i misread your description… After thinking about it, i would still need the wooden frame to attach the adapter plate, since there cant be any fiberglass over the adapter.
Keeping to adapter fixed on the bottom might still be nice, because then the vesc would stay permanently as well inside the board.
I guess i will try the most simple version first and then learn from it and see what can be improved…
Do you have any recommendations for a connector from the motor cables to vesc? Small but capable 
You will connect/disconnect them when removing the mast, right? Waterproof?
Correct. I realized the cables already come.with banana plugs, which would be an option. I guess a waterproof comnector never hurts, but there should never be any water in the compartment
Im not familiar yet with “real” waterproof connectors. But a buddy recently made a damn waterproof one designing it around a M20 metal cable gland which uses 3x phase 4mm bullets (the good ones) and 7 pins for low voltage/data. We use it in Onewheels pulling up to 250A + peaks.
But the foil motors using AWG8 i guess 6mm bullets could work well if compartment itself is waterproof.