that is a pretty good idea for the exhaust diverter. When i was experimenting with mufflers for my saito the Davis Diesel muffler was the same idea of mouting as your extention. It had a brass inlet tube which had slits on the four corners, and you mount it to the header of the saito. Then you use a hose clamp and clamp it down on the header. Never tryed it because i would require lots of hacking of the cowl to install it. More than stock.
I do have the dubro rubber diverter and i zipped tied it on the muffler.. Seems to be really snug. But, when the oil and crap starts shooting out the muffler when its runner may cause it to slip off.
Anyway its time for me to start up the motor for the first time. And start the long awaiting break in process. Im going to leave the cowl off for a bit so i can get to the needles when needed. Give it a couple good tanks on the ground before setting it up in the air. Dont want a deadstick on a mainden flight.