You might look at how a flue exits a roof, there might be some application that you could put into a topper to fit your application. Depends on the temp of the stack where the topper would make contact you might not even need the special boot for high temp applications, you might get away with a rubber boot that would slide down over the stack and form a seal. My nieghbor has a transfer tank and put a short box topper on his truck behind the tank and just built a small plywood wall to extend the topper front to the bed of the truck. He painted it to match and sealed it with Buytal caulk for flexibility, looks pretty good and he has the function of the transfer tank and dry storage. Congrats on the truck!