I bought butyl tape at a building supply. It's used to seal metal roofing panels. It's thinner (1/8") and narrower (1/2") than what the rv store sells. For width, I doubled up. For height I stacked it. I've done windows
, roof fan, water inlet, etc. No failures. Best done on a warm sunny day. Tightening the screws causes the stuff to meld and ooze out the edge. Trim with a dull putty knife.
The stuff is sticky and attracts dirt. The last time I used it was the roof fan. I sealed the edges with Lexel to keep it clean. Raz