I welded 1 3/4 exhaust pipe onto the muffler end, it was a good bit larger so had to fill in. Then took a 90 degree exhaust and turned it down to adapt it to 3" double wall gas vent pipe. This was to go through the plywood floor.
I ran the machine for a few minutes like this to see how hot the double wall pipe would get before I cut a hole in the floor, well it got really hot. Too hot to be around plywood.
So I am left thinking of putting a collar on the 3" double wall, cutting the hole through the floor oversize with an inch clearance all the way around.
But the collar will be attached to the hot pipe and transfer the heat to the wood that way.