How about brushing a coat of epoxy onto the inner surfaces? That should at least act like an ablative coating. I typically use CA on cardboard motor tubes if they are standard tubes, figuring I might be lucky and actually have the rocket last long enough to worry about motor tube burnthru.

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Lee Reep
NAR 55948

Projects: Semroc Saturn 1B, Ken Foss Designs Mini Satellite Interceptor
In the Paint Shop: Nothing! Too cold!
Launch-Ready: Farside-X, Maxi Honest John, Super Scamp