I use a Wren single wall pipe with a PST 600 (54 size engine) in a Trim Sabre. The fuz does get very warm by the time the model is ready to taxi out. After taxying back in from flight the fuz is cold due to the airflow through it, but does warm up again after the engine is shut down, due to residual heat in the pipe. You will need to apply heat shield such as ceramic blanket to any cables or pushrods that are in the aft fuz. I use a Wren pipe with a 44 in a JHH Skyhawk but for that I made an epoxyglass outer pipe coated in BVM heatshield paint and it seems to work well. The problem to watch out for is that the Wren 44 is very sensitive to the pipe, and it must have a divergent duct at the start. If the JHH twin wall pipe does not have the divergent duct you may have engine running problems.

H.