If I could afford it I would use a seamless 304 Stainless Steel. Probably cost a bit and hard to bend but not as hard to bend as the 400 series stainless.
My fall back would be an Aluminum tube. These two would be easiest to keep clean inside and out. (also hard to bend depending on alloy and heat treatment).
You might want to give these guys a call, I wouldn't tell them what it is for but ask them about getting a 3 or 3 1/2 inch OD tube, 2 feet long bent the way you want and see what they say.
Coltwell Industries Inc.
55 Winans Ave., Cranford, NJ 07016
Tel: 908-276-7600 • Fax: 908-276-2679
Email:
info@coltwell.com
BTW I don't work for them, I have no affiliation with them that I know of. They are the first ones I came across doing a google search.
All the materials you mentioned powdercoat well.
A polished stainless will reflect a lot of heat. The Al will transfer heat a little faster than the SS or Mild. But if you powedercoating there shouldn't be much of a difference.
hope this helps.
I would jokingly refer you to Carbon Fiber if your looking for all out insulation.