Okay, let's look at it this way:
- If you coat the receiver board with epoxy, you have to take it out of the case anyway AND you make the receiver unrepairable. Epoxy also adds a lot more weight
- I get the room issue, could be done, if you were building the boat at the same time
- I'm thinking water is getting in through the drive shaft tube, the rudder pushrod seal or something you didn't think about, through the screw holes in the boat's bottom, that's assuming your repair got all of the damage. I would make two changes, replace the paper towel with a cut up kitchen sponge and fill the screw recesses with silicone caulking compound, just in case