one of the basic black metal mesh tank lids
This could be one of several things, each requiring a different approach - how about a pic? And, are you the least bit handy? You mention owning a Dremel, so clearly you've got an open attitude (some people are voluntarily helpless; I don't understand that). Do you own a drill & carbide bit set?
I'm thinking you "just" need to modify the existing lid into a "fat C shape" and build a door that fits onto the open part of the "C". Get yourself some little hinges and attach the old and new lid parts.
It would be easiest if you have an aluminum-framed, window-screen lid with fiberglass splining. In that case, if it was me I would just buy a window-screen repair kit and some extra corner pieces. That, and a pair of small hinges (or a tiny piano hinge) and some small metal screws to fasten it to the lid and door. You'll also need a splining tool (looks kind of like a pizza cutter).
Second easiest would be if your lid is stamped or perforated rigid metal. Just carefully hack out a door, and rivet some hinges. Actually, the order there is backward - rivet the hinges on first, then cut out the door. You may have to destroy 4 or even 8 rivets in the process, but your door will swing more freely and it will definitely be easier to get the hinges on. Rivets and riveters are cheap. There are many, many, many
uses..."what couldn't be improved with a few rivets???" ha ha ha.