The dimentions on a 12" are 12&3/4" O.D. 11&7/8 I.D. with a wall thickness of 3/8", these tubes are used as cores for a local pulp and paper mill, they sometimes have pressbord plugs in the end with a hole in it so bulk paper can be rolled on to them then unrolled off of them and fed into various paper cutting, milkcarton making, toilet paper rolling type devices. Basicly a 12" OD tube could be used for an 11&1/2" mirror if the mirror cell was made to adjust in that tight of a space.
I have some cut off ends that could be mailed fairly inexpensively if you wanted to look at the quality of the cardbord and check the dimentions. I can get pieces up to 72" long for free if I catch them before they get to the dumpster. (if there is any defect in the total length (20+ feet) they cut them in to short lengths and place them in the dumpster for a recycler. Longer lengths can be had for a price I`m sure. I have quite a few 12" tubes in my shop right now. I try to get them until they get tired of seeing me then I wait a while and go back. I get to being a pest I may start getting charged for there waist ;) . Give me a call if you would like to have me send you a short (1/2" to 1") long piece of 12" tube.
Work: 1(208)746-2663 (Kight Dental Lab) or Home: 1(208)924-6828.

P.S. These are not tappered like the Home Depot concreate forms.

Clear Skies,
Tony