After having let my stock tails thoroughly dry (to remove all the condensation inside) and making/installing my new gaskets (
thread here), I can happily report zero internal condensation/water. It's been raining pretty hard all day and no sign of anything wrong. The only thing I have yet to test is the car wash.
Before installing the custom gaskets, there was consistently condensation/water inside the tails every time it rained even a little bit.