The rubber o-ring size (to replicate the 625 battery) is a #9. Works well in many devices as long as the device doesn't rely on electrical contact with the side of the battery.

I've seen (and have) metal washers. They do allow electrical contact on the side (here's one example: I believe Jon Goodman's adapter is more than just a ring; his design might be preferable.

I have one of the adapters made by de Gruijter. It was a bit cheaper when I bought it (but everything was cheaper than). It, like the Gossen adapter, is the "best" solution as it uses silver oxide batteries.

The limited life of the zinc air battery can be an issue. I think it somewhat depends on how much you use the meter. If very infrequently, you can try recovering the holes with electrical tape. That supposedly inhibits the chemical reaction and lengthens their life. If you use the meter every day, then it's fine to use the zinc air. They're inexpensive and you'll get good use out of them. It's the middle ground, where you end up getting relatively little use out of the battery before it dies, that I find awkward.