Well, I tried to fab up something similar to what Crob42 fabbed up, and well, it didn't go so well. Most likely it's the drill bit I have (it's a HQ Norseman) because it bites too hard, but the derlin blew apart on me.

So, plan B. I did also buy the silicone washers for this very case, and this is what I did, from bottom to top.

Butyl around the white plastic
Silicone washer
Prinsu spacer

On the bolt, I slipped on the supplied fender washer, then a 1/8 buna o-ring (-203), this should seal up the void between the bolt head and the spacer so water doesn't enter and pool in the spacer.

Edit: I applied red loc-tite to the bolt.

Cranked it down, finished my beer, now hope for the best.

I may get spacers fabbed in the spring.