1) I used a 5/8" hole and channel, and a 3/16" gap around the tenon. The polyurethane is very fluid when first mixed, so these sizes were more than enough. Make a dam of modelling clay around the pour hole and the mast, so that the cavity can be slightly over-filled.
In Canada, I used Smooth-on brand, their TASK series, but I can't remember which grade.
2) I made a cable exit hole just above the heel plug, and this would also act as a drain.