I found 12" stainless screw "hose" clamps locally at a plumbing supply wholesale house.
The clamps are are sold by the corresponding nominal diameter pipe size.
Do the math
3.14159265358979323846264338327950288419716939937510582097494459230781640628620899862803482534211706798214808651282306647093844609550582231725359408128481117028410270193852110555964462294895493038196442881097566593344618475648233786783165271201909145648566923460348610454326648213393607260249141273724587006606315588174881520920 96282925409171536436789259036001133053054882046652138414695194151160943305727036575959195309218611738
x12" = 37.5"+ tag end
They do make them larger. Search "Fernco Clamps" and see a 14"-16" size - product # 248-300
http://www.fernco.com/plumbing/flexible-couplings/stainless-steel-band-clampsThat should do the trick. Any plumbing supply house worth a drip can special order Fernco.
Call your local Grainger branch, too - may be a special order.
Another option is the strap used to hang pipes- 1/2" wide, galvanized and perforated that Jim posted - cut it to length and use a nut and bolt.
http://www.oatey.com/Channel/Shared/ProductGroupDetail/12/Metal+Hanger+Straps.htmlFound at the Oatley display in your local Lowes, Home Depot, Tavern/Post Office/Hardware Store.