I'm a little late to the party, but ...
I was unhappy with the seal provided by the fermonster o-ring as well. At the same time, I tried the mesh strainer for dry-hopping and found that the strainer's top - which sits on the rim of fermonster's mouth - acted like a rubber gasket and gave me a...
Brewed an APA, a staple of summer for me. 2-row, vienna, a little crystal 40, and a bunch of Cascade.
Decided to do a split batch: WLP001 Cal Ale yeast in one, BRY-97 in the other. We'll see if I can taste the difference.