freeze proof a drain trap which is now exposed to the weather

[das fisch]

so i took my two boys up to camp yesterday for the night to get away and chisel away at some finish work. temps dropped from about 40deg to about 0 if not a bit lower by midnight. woke up and found out the hardway we had a drafty area up in a knee wall which froze a small section of pipe. got that insulated tight and ventilated to the warm side and quickly thawed out.
then when shutting down the water noticed anitfreeze wasn't going down the tub drain. trap fully frozen! faux pas on my part I guess.

any how the trap is below floor level on a post/pier construction. i need to weatherproof it pronto, or at least after I replace the trap with one with a screw in drain port. would like to maintain access if need be. possibly boxing it in and insulating heavily with some air circulation from the warm side.

any one have a similar experience and ideas?