Here is another option and how I installed my fan -> https://sprinter-source.com/forums/showthread.php?t=23817My plan is to install the Maxxair fan in the location well described by Dave, next to the second from the rear rib. A couple of month ago I talked to Maxxair and was told that their unit needs to be installed on a flat roof which is not the case with the Sprinter. I just measured roof curvature and across 16 5/8" fan's base flange there is 7/64" (0.11") gap on both ends. The roof is right next to the rib so the roof's sheet metal will stay curved at least at the rib.
I am thinking to do one of the 2 options:
1. “Shim” the gaps with additional butyl tape.
2. Make a 16 5/8" x 16 5/8" with 14" x 14" hole 1/4” - 1/2" PVC or HDPE frame with corresponding curvature on the bottom and below the roof a wood frame with curvature on the top. Orion used Styrofoam to compensate for the roof’s curvature and corrugating stiffeners. PVC has 1/2 of HDPE coefficient of thermal expansion.