JVB - the suggestions you got from others are fine, IMO. I've actually made an entire "header" tunnel before by gluing in a paper towel cardboard tube, then coating with epoxy. Tunnels and such shouldn't be structural items. If they are, then just a couple of light partial bulkheads are all that's needed. Coated with epoxy works. I've just gotten my can of Liquid Heat Shield by Bob Violett models (from Central Hobbies). It's supposed to be a brush on heat barrier like space shuttle ceramic; I know the jet guys use it. I'm putting some on the Excelleron wing/tunnel area, just to be safe.