I made an exhaust stack for Dad's motorhome tonight.
First attempt was 2" aluminized exhaust pipe from the local muffler shop & a long radius sweep elbow (EMT?) slipped over the generator exhaust. Dang thing was heavy.
We welded a hook on the top to fit into a bracket added at the roof line. This pulls in air at the bottom, but not enough & as a result, the stack temp was over 250F.
Use 2 down spouts, a 2x3 inside a 3x4. Outside temps never got over 110F.
But the dadgum thing made more noise than was tolerable outside the coach.
Looks like I'll be trying some more things as time allows. . . .