It's heavy duty plastic, at least 1/4" thick. I certainly hope I couldn't crack it by tightening it with my bare hands...! Plus both of them? :S

I can see the water slowly build up around the threads in the lid... It seems like the seal isn't tight enough...

the threads do not seal, the O-ring does the sealing, i have purchased 4 of that type canister from BRS and have found that there are 2 different threads, they are very close but you can't get them to seal if they don't match.
also i have had to tighten them with the wrench sometimes, they are very strong so don,t be afraid to crank on them a bit