If it is single wall any damper will work. If it was me i would get a cheap peice of snap lock pipe and put the telescoping section up higher just to try it before drilling an expensive piece of pipe. See if it works and if it does help then put it in the telescoping section just make sure you put it in a spot that still allows the pipe to collapse enough to get out
ok that makes sense to me. since i use the flue temp to judge when to start backing off the air for secondary burn, i guess i need to see if its possible to install it up above the 18 inches where the thermometer sits? or just move the thermometer and readjust the way i read it, since it could be higher or lower depending on where i move it, or on which side the damper is from it.