I use the tapered silicon plug on my Victoria. I have a wire through it with the wide part of the plug inboard. Such that all I have to do is push the plug in tilt the boat drain the water and then a quick tug on the wire and the plug seats itself again.
on my two IOMs i drilled a small hole in the transom mid way up well above the water line Which as you mentioned makes accessing the rudder post screws much easier. since the hole is so high you don't need to seal it with tape.
As mentioned above the old school water gun plugs are a popular idea as well
Honestly I would not overthink this. Sometimes the simplest method is the most elegant and best
the picture of the plug by itself is next to a sails ect large bowsie. so you get an idea of the size...