I've been using the over the counter alkiline replacement batteries in my old meters n have no problems with my negatives or readings. I think if you just standardise your meter to your film, you will find the meters are very accurate n reproduceable. If you keep comparing meters to other meters, even the best of em will not agree msot of the time. Meters are for exposures, not racing against eachother. And BYW the battery last so long it will take several years to experiance the voltage drop off point. Rplace your battery regularly and you won't have any problems.
I like that voltage regulator circuit in the FX3 idea but still takes up too much room.