mag can run externally
I personally would get the tank drilled...but thats just me, i have seen planty of 20 gallons being drilled, is can be risky but once its over with it helps out alot....and if it doesnt well in this hobby the price of a 20 gallon is probably one of the cheapest expences.
Have you figure out if you are drilling or not?
IOf not check out melevs site, and look at the closed loop w/SCWD...no drilling for this closed loop.
But to the answer to your question i would use a mag 7 and they can be run internally or externally.