Yes I do have an external muffler under the belly, I had it custom made here in Aus with Inox as a baffle and so far has worked well and held up with the heat.

The rest I welded up a simple stainless exhaust then made a shroud under the cowl for it and piped air into it with the outlet towards the back, has worked well to date.

I tried flying without and with the muffler and made no discernible differance to speed, but definitely noise factor as you'd imagine.

Hope this helps

Cheers

If I could upload pictures I'd send you what mine looks like.