What I did here was to just place the collar on the bottom barrel and center it. Using a nail or scribe, I marked the inside of the collar on the barrel.
	OK, here is where you make your money with your saber saw(sawsall). It does an excellent job of cutting out the round holes for all the vent pipe connections.
	Once the hole was cut in the top of the bottom barrel for the vent damper, just drilled the holes for it and bolt it in place.