I am using a plain, home made epoxy glass pipe as the outer on my Skyhawk with MW44 and Wren stainless steel inner pipe. The outer pipe is made from one layer of 200gram cloth cured flat, painted on the inside with BVM heatshield then rolled into a tube. It only has approx 5mm gap to the hot jet pipe, so far it has not shown any problems even with extended ground running with barely any airflow through it.

H