[chromo]

I don't know about fumes or such.

What I did for condensing vapors like that and sealing those tanks was use that caulking putty tape stuff like for around doors and windows on metal buildings or trailer houses. It stayed sticky and I could form a round strip with it. I ran it all the way around the top of the tank and also plugged a few holes that were near the handles.
For my lid I had some heavy clear vinyl, factory scraps I think were from table covers. Other heavy plastic sheets should work well also.

I put the plastic sheet over the tank so it would hang low in center to drip then pressed it tight to the putty to seal it, another try I had raised the center so the condensation would run out to the sides and down into tank. Both ways were well sealed and did not make any mess for me. Since you don't need to worry about seperating tanks you could probably do about the same thing but snap the tank top back on above a plastic sheet, or maybe just use the top and the putty.

What I was doing was evaporating liquid from one tank and condensing into the other, a tank in a tank, so I had to direct the drips where as yours can drip back into the tank anywhere.

What stands up to the chemicals in use is always a question. There is that soft rubber weather stripping also that might work. If your tank is like mine was then there are a couple holes near the molded handles that need plugged up and the top fits loose. If the putty and a rubber strip will hold up in use it should make a decent seal.