Thanks Jim. I bought the flap material at a truck supply place here, San Antonio Brake & Clutch. Each flap for an 18 wheeler is about $15 as I recall, and they are mounted sideways. The "material" is the same stuff the bed mats are made from....looks like revulcanized ground up tires.
My receiver hitch is the original GMC piece, but I did not like the extra "slop" allowed by the 2 1/2" receiver with the 2" reducer. So I went to Northern Tool and bought a blank 2" receiver tube, cut the end off of the 2 1/2" receiver, and welded the 2" tube into the cut 2 1/2" tube, creating a super tuff 2" receiver tube.
The "trumpet exhaust" is actually a cooling exhaust tip designed to cool the exhaust when the system has the Diesel particulate filter in the regeneration mode, and actually does a pretty good job. The exhaust temp is such that it will not damage the flap, even blowing directly on it. Especially if a certain "modification" has been done to the exhaust system.
PM me if you want to know more.