Does anyone know where I can find an exhaust mounting component that wraps around the entire exhaust, and then can be attached to the body. The whole thing looks like a giant P-clip.

One of these, but of course in the UK:
http://www.plazacycles.com/Stainless_Steel_Replacement_Strap_and_Rubber/2,8,3505

that is just a silencer strap used on motorcycle exhausts

R1 cans come with them in stainless, I am sure other bike exhausts have them too.

how difficult would it be to make one from a 20 mm strip of stainless?

Just search eBay for exhaust strap several on there at the mo