Without doing any real research, have you tried a 2" PVC threaded cap?
Our Nissan forklift at work has a plastic PVC home depot plumbing section cap on it to cover the gas filler. (yes, it is an old bastard POS) I was surprised it fit, especially being a japan built piece of equipment. Screws right on and doesn't look like anyone has monkeyed with any of the threads.
Might be something to check.