I made an adapter for my Leica MR-4 lightmeter (which requires only 1 cell) using the instructions I found on butkus.org. I can't seem to produce the link, but it downloads a file called batt-adapt-us.pdf. It was written by Frans de Gruijter in The Netherlands, but I am not sure if the original author still has a web page up. At any rate, it discusses uses Schottky diodes to reduce the voltage of silver-oxide batteries. Quite a lot of good information there, but you have to be happy using a soldering iron to make one of the adapters using the silver-oxide batteries. At the end of the documents is some info about buying from him, but I am not sure he still does that so you have locate and make everything yourself. Some of the commercial adapters are just to 'fit' a modern cell into the same space and the voltage of a silver oxide battery will be wrong compared to the old mercury cells. Apparently the zinc-air cells used in hearing aids have a closer voltage but a more limited life span. Look at the de Gruijter document, it explains quite a lot and the adapter I have makes my MR-4 meters work.
Following that info, I also soldered another diode into my Lunasix meter but I'm not sure it all worked out OK because the battery test doesn't work, though the meter does. In the end, I bought a Digisix meter, reasoning that it would be good to have a meter that was designed to use modern batteries for when the MR-4 didn't fit my needs (like another camera.....)
Jonathan