viz:
http://tinyurl.com/mb4jqph
http://tinyurl.com/k2zbnoq
http://tinyurl.com/lk87r54

It's Schedule 40 PVC pipe and I figure I'll just put a bead of white
silicone caulk around on each side and sort of knead it in so if fills
the clearance gap.

Exhaust heat is not an issue: I left the device running for an hour with
no hole in the door and the muffler 2" from the wood and the wood was
barely warm. I can hold my bare hand up to the exhaust no problem.

Main function of the caulk is to keep the PVC from falling out and maybe
take the occasional bump - but nothing more.

Since the caulk I have won't go off until temps are in the sixties I've
got plenty time and figured Greater Minds Than Mine might know of
something more appropriate.

--
Pete Cresswell