I mixed up a batch of resin, hardener and 403 filler. I filled the void in the transom drain hole and the stringer as well as the space left open by the removed portion of the stringer. Then I wrapped the new tube with 2 wraps of cloth wetted out with the filler mix. Shoved it all through the transom from the rear and spread out the cloth inside the well to make a decent seal. once this was done, I tapped the tube from the rear with a hammer and smoothed out the resin/cloth once more inside the well.
Ready for glassing the well with cloth & then paint.