I do think it is a very a good idea to have the drain feature somewhere, even when using a cover. Protects the wood from long term pooling of water, as now matter how careful you are, there are always "accidents" that cause a cover to blow away or sag so badly it collects water and dumps it in the boat. A drain makes it easy to wash the inside of the boat, if nothing else.  An Anderson (auto) bailer would be my choice if you do the drain in bottom of the (thin) hull, even though you'll rarely be going fast enough for it to be useful underway. But the one argument is that on the days when you are shipping water, those are the days when an auto bailer might actually do some bailing for you. The good Elvstom design stainless steel ones that inlcude a lever arrangement that locks them shut should leak none or hardly at all when the gasket is new. https://www.westmarine.com/andersen-mini-automatic-bailer-13561022.html

I put my plug in the base of the transom on my NE Dory, and I do raise the tongue enough to ensure it drains.  You can always hang a concrete block over the trailer tongue - or some other sort of tongue weighting arrangement if necessary (weight isn't needed for my dory trailer, but I do use a block on my Flying Scot trailer).  And if you're really worried about wind, screw-in tie-down stakes with a strap that runs over the boat might be an option, so long as you're always able to park in the same spot.  We did always strap down our boats (various sailing dinghies) when I was in Kansas.  And we just had a microburst (so bad it took the roof off the clubhouse, just like it was a tornado, maybe it was - it was very localized damage, but the bullseye was the club grounds, go figure) here in Georgia that damaged several boats buy flipping them off of trailers.  Here almost no one uses straps.  Actually, there hasn't been a rush to strap down boats here in the aftermath of that rare event, either.

Thanks Bubblehead!  Despite my previous not-so-great experiences, I may end up going with that mini Elvstrom/Anderson.  But haven't given up hope that someone has a simpler solution.

   Andersen drains do often leak when closed. But usually not much and once you get underway it drains out.  Provided you get enough speed. Our friend Mark Nye reports it takes 4 mph to get the drains to work right.