I used Gorilla Tape on a shower skylight that had a big crack/leak. Also, the old reliable, J-B Weld will work if you need to leave it for an extended period of time. I'm surprised that Fantastic Fan cover would break, but have you contacted them directly? I did when one failed and a complete brand new fan landed on my doorstep the next day. Yes, I am aware that Dometic purchased them but understand that they are still good to work with.
Dave W along with my DW, Susan and our poodlepups, Callie & Molly,2011 Ford F250 6.7 CCLB, 5er Hitch Option w/B&W Hitch,,Ride Rite air bags, 2014 Montana High Country 343RL (38')