I'm no plastics expert, but my experience has been that just about any plastic will melt in contact with a hot muffler. A little airspace will save it, but contact will melt it. If not airspace, find something that can be fit in somehow to provide insulation.

If you go for bare metal, be absolutely sure you have clearance between pushrod and muffler through the entire range of throttle up & down. I had terrible interference once from a metal pushrod inside a plastic guide tube. The plastic would touch up against the muffler, finally melted through, and the metal pushrod would scrape against the muffler. Every time I'd touch the throttle, the plane would throw a fit.

I have learned to be very careful in routing my throttle pushrod. I fit a piece of wire just long enough to run from throttle arm to firewall, clevis it to the arm, move it around with the engine mounted to see just where I want the pushrod to come through the firewall. With some mufflers, plus tank clearance to watch out for, the open spot can be very small.