The MR-9 adapter from the Small Battery Co is the way to go...
https://www.smallbattery.company.org.uk/sbc_mr9_adapter.htm
When I tried an SR44 in my Rollei 35S, I got all sorts of weird readings. An ASA setting that gave me a good reading in one subject lighting , gave something incorrect in another. I had read about the internals of the camera and found that there are two adjustments inside - one for the overall setting, and one for the slope or curve of the setting. In other words, you can adjust for the voltage of the battery, and then adjust out the tendency for the reading's accuracy to change as the amount of light changes. In the end, I never bothered and just used the camera with no battery. Then, I decided to splurge on the MR-9 anyway and found that it basically does what is says on the tin - set ASA and point camera at subject, and the readings are correct (ie, match other meters). It was pricey, but it was painless and it works.
A note on batteries...
The MR-9 comes with a 386 battery. This is supplied because it gives a package that's close to the original thickness of the original PX625. However, an SR44 will also fit, albeit with a bit more height. It so happens that the battery compartment in the Rollei 35 is fine with the increased height - it seems to screw down normally, and there is still room for the film cassette to fit with no snagging or friction. I think the upshot is that the SR44 has a lot more capacity than a 386 (about twice the height), so should last longer.
Don't use alkaline
LR44s - as the power depletes, the voltage drops. The Silver Oxide
SR44 maintains its voltage until it's close to drained. I don't know whether this matters in the MR-9 adapter, but I'm pretty sure an LR44 can drop below 1.35V before it's drained, which I think would mean the adapter's output voltage would also drop. (I don't think it can magic up more voltage, but if it can, it's probably at the cost of using more current, meaning the battery will start to drain faster.)