Though I'd show off how awesome a hole saw is for cutting out tower holes, that is if you're using a round tower. I bought a 2" deep hole saw, and it wouldn't go all the way through the frame rails, so I finished the cut with the trusty sabre saw, then ran the hole saw through the hole to bore out my crappy sabre saw cuts. A little grind here and there and the tower slid in nice and tight.
I was worried I may have to buy a beefier drill to use the huge 6" holesaw, but my old 4.5 amp black and decker drill cut through in about 2 minutes!
Hole-saws... a viable option!