Locking Lid for Freshwater tank

There do exist some plastic covers for such things, and you can come up with a solution.  Can you please describe the problem you are trying to solve?

Well, i guessed that!   Are you trying to secure it just to keep things from popping in, from malicious folks trying to poison the water supply, from losing key pieces, etc.?  Different reasons lead to different solutions ...

Pretty handy looking!

I don't suppose the fresh water filler is the same size as a gas cap for a car, is it?  I'm doubtful, but if you could find a locking gas cap on Amazon or from your local Pep Boys, that would be a very easy and cheap solution.

Add me to the list of those curious about why you'd need one, though.  If it's to protect against someone poisoning your water, you must either live (and/or camp) in a really tough neighborhood, or else you have a very angry ex. 

It seems as though, a while back, I saw a door that installs over the existing fill port. No disturbing the existing plumbing. When I get the time, I'll do some more searching.

A few years back, one member (upon returning to his campsite) caught some young cherubs filling his tank with rocks. So, things do happen.

In my case, the cap lanyard lasted about one season, the bug screen for the vent tube is now missing, and after over 6 years of baking in the sun, my fill port is kind of crappy looking. Since it needs replacing anyway, a little extra security won't hurt.