As a former Hunter dealer who sold the 36 but now retired, I find it odd to drill drain holes in a rudder that will remain permanent with the holes being plugged and unplugged to get moisture out at the end of each season. With the paticular construction of the Foss Foam rudder, I would tend to think it is getting moisture or water into the rudder which would need to be addressed from leakage. I understand why the boat would come out end of the season due to location and climate, freezing. I would have reservations as to permanent vents As for the rudder, I would look at the seams for the two halves and where the rudder post goes into the rudder. I am not sure if the rudder has barrier coat and so forth. Since I do not see any pictures, I use to drill holes in the bottom of rudders and let stand for a long period of time allowing the rudder to dry out for lack of terms. Then I would either fiberglass also using a two part epoxy called Water Tite or Water Tite alone. Sand and then apply barrier coat and bottom paint.
My best advice is go direct to Foss Foam or NewRudder.com and that telephone number is 352) 529-1104
and speak with Bob Walker. They built the rudder for Hunter.Go direct to the appropitate source would be your best bet in this case.