Just a quick update. I made a small board out of strip board and placed a holder for the coin type Lithium batteries that are so common (CR2032). I put a schottky diode in series with it to prevent the reverse charge current.
I'll just put a photo here and then if you're interested you can read the rest of the boring details :)
First I tried a bigger battery actually but the holder I bought for it that was supposed to be able to hold it, was too shallow, so the clip sat too low where it was bent, preventing the battery to be inserted all the way. Annoying. So, that's why I used the smaller but more common CR2032 type. Lucky I bought a CR2032 holder+battery at the same time, just in case the bigger one wouldn't work for some reason...
After struggling with my useless little engraving machine (like a Dremel but wobbly and weaker, and cheaper..) I made a hole big enough to fit a screw through. I drilled a small hole in the bottom of the wooden case (yay for wooden cases!) and screwed the little battery board in place. Finally I attached the battery wires I had prepared earlier by putting a simple pin-header on them to a socket I placed on the battery card. I made it with three pins, using only the middle and one of the outer pins, so that if I put the connetor backwards I won't reverse the polarity. Of course I could still reverse it if I let one pin sit outside of the socket, but hey, it's a small safety precaution at least. I could have used one of those polarized connectors but I don't know if I had any at home and I already had wired soldered in, so.. Bah. This should be good enough.
The battery is a 3V cell but I measured the cell unloaded and it said around 3.2V (don't remember exactly) and when measuring unloaded after the schottky diode I even got about 3.07V. I hope that will be enough. Maybe I should measure the voltage when it's connected too :)
Now, all that remains to be seen is if it holds the patch memory after being powered off. If it turns out the voltage is too low I can always replace it with an NiMH battery instead I guess. At least it's easier now that I have the wires attached so I don't have to remove the CPU board.
Oh, it also remains to be seen if the battery explodes when the synthesizer is plugged in... We'll see :)
At least if it explodes it will do so away from the precious CPU board...