Jon,
I've used the thin, clear, plastic that comes on the top of greeting card boxes... made a wooden jig to the proper scale, then made a clamp jig.
I cut the clear plastic to the shape I desire, wrapped it around the jig, heated it with a hair dryer, super glued it (on the inside using a thin strip from a sandwich bag to cover the seam and act as a bonding bridge), then attached the clamp jig around it, with the seams opposing.
After a couple days, remove the clamp jig, and sand in filler putty with "0000" steel wool along the seam. You can add additional strips along the section to simulate thicker area's on the pipe.... exhaust blueing can be accomplished using graphite after painting, but DON'T remove the inner jig until ALL work is complete.
Also, greeting card plastic is great to fabricate panel fairings, or to Vacuform fairing blisters, ie: ADF Loop Antenna Fairing.
I have illustrated how to accomplish these techniques, and can publish them if interested.
The 1/32nd "G" Model I converted was during the time of the first "P" Models, so none of the thing's you're doing (exhaust cowlings and IR suppressing exhaust) to yours existed then.