What can I say - I'm from the old "make do or do without" school.
But obviously not from the mechanical drawing school.  

A pair of these could be made to fit most any older engine having an exhaust stack wider than the crankcase.
Cut from extrusion if appropriate stock is found (storm door/window frames? Boat trim?), from aluminum stock if a milling machine is handy or even cut/bend from sheet with soldered tubing for the bolt.
While there are many homebrew designs, I think this looks rather neat and shouldn't deform the cylinder as some can.
Maybe someone will give it a try, or develop a better idea from it.

regards,

Terry

This worked for me.
Take two pieces of brass rod, 3/16 inch long, drilled 4-40 UNC.
Cut the appropriate length of .025 x 5/16 brass sheet.
Silver braze the two threaded pieces to the brass sheet as shown.
Put two thicknesses of electricians tape on the brass sheet, where it contacts the bypass.
Wrap the sheet assembly around the engine, secure the muffler with two 4-40 bolts and you're all set.
This setup has survived over 600 flights, never adjusted or tightened.
Bob Z.