Thanks again for the help everyone!
I love rescue tape but didn’t see the post in time. If that works it seems like a nice, mess-free solution. But I’ve had trouble making waterproof seals with it in the past. Ended up with a tube of Sikaflex. And I’ll definitely look into making a cover.
Launch was Wednesday, but when we went to step the mast there was 1/2” of water in what had been a bone-dry bilge the day before. Uh-oh. Found a steady trickle coming around the thru-hull for the speed transducer. So I got to do some below-the-waterline sealing too! Happy to report that (after much work) the 2nd attempt yesterday was successful.