This is an easy way to make a mold for a muffler shroud layup.
I pegged a board which I clamped to the table of my band saw. Using 2-inch blue construction foam I cut square pieces which I mounted on the peg and pivoted the pieces about the peg through the blade ending up with a stack of foam discs. A bit of glue and I had a cylinder around which I could do layups to form a U-shaped form.
Dissolve the foam with lacquer thinner, cut the layup to shape and bond to the fairing over a precut opening which allows the fairing to be mounted over the muffler.
Layers of cloth and resin secure the shroud which is then ready for finishing.
My muffler is the Powerflow four-into-one unit built for the GlaStar.
I haven’t flown yet, but based on comments from builders, the Powerflow exhaust is supposed to have noticeable performance improvements for the O-320 Lycomings.