Standard brass tubing, epoxied into the transoms, would work for drains, sealed with radio box tape when running. They are what were installed in the rear of the "shoes" of my scale Elam and yes, they work well. IF your boat was still being built, there are other options you could have used but, with the hull being more or less finished, you don't have too many options. One question I have is did you make any changes to the under-deck frames or are they still solid panels. There is a reason I ask, which I will go into later