I am also using the OS .50 SX,with the supplied muffler,and an O.S.extension.

I attached the cowl permanently;a bit more work but it looks better and no screws to back out.I moneycoted about 1/8th inch over the edge of the cowl,after applying a bit of Stix-it.
Then masked with 3m low tack tape [the olive green stuff]and painted the cowl with TF Lustrecoat.

It's ready to fly in a couple days,but probably won't until Spring.

Dave