On my Ephesian twin boom ship I formed the bottom of the center fuse' as a balsa tube and the rest of the ship is basically above the exit of that tube - stayed wonderfully clean.
Funny this should come up at this time as I just sprayed the final coat of regular butyrate on the open bay areas of the new Ephesian now under construction for my PA75 and will start spraying the non-tautening dope on the whole ship on Monday!

Weight at this time, including the power train but not the flaps, cowl, and elevator is at 46 ounces with 715 squares of wing area.
As soon as I get the blocking coat on I'll start a new thread with it.
But for now, that ship, as well as my two Crosswind stunters stayed really clean with the "out-the-back" exhaust. No extra tube, you understand, just a large jet like tube for the lower fuse' section.
If I'm successful in attaching a picture of my two Crosswinds you'll see what I mean. (I haven't tried the picture sending thing on SH yet, but here goes. I was too lazy to remove them from the rack so they are as you see them upside down.)
Blessings,
Will