I believe it has to be an outside circumference of 29-5/8 inches to be big enough for a bowling ball mortar. The wall thickness on all of the tanks this size (that I've cut) have been slightly thicker than 1/4", more like .30". There could be some of this circumference that are difference, but I've cut 4 of them and they were all approximately this size.
Hope this helps.
NitroSteel