Rainy Day Fantastic Fan Installation Blunder???

I think you did yourself a favor.

I used the foam gasket and had a leak.

I used dicor and had a leak.

I used dicor and ha a leak.

I used butyl tape and it's been fine ever since. After about a week of letting the tape quit oozing, covered the screws and seam with a dicor self leveling. The dicor cracked after a couple of years, but part of my annual routing is to paint the dicor seams with an rubber paint which fills in the cracks.