I was just getting ready to suggest contacting Frontman for the battery cap, but he beat me to the punch.
As for the internal wiring getting corroded, that is a possibility, but not un-fixable. It's just a matter of replacing the wire from the top of the battery compartment to the meter. A little bit of effort, but not too complicated. A battery powered Weller soldering iron is real handy for that job.
PF