I have a number of classic Mac computers that use a 3.6V 1/2 AA sized lithium battery to keep the system settings and clock. While these batteries are only about $10.00 CDN online in 2019, I had some AAA battery boxes available from a MicroBit kit I had lying around.
It turns out 3.0V from two AAA batteries is enough to keep the time ticking on these older machines. I have access a 3D printer at work so I built an adapter in the shape of the original battery that has a channel down the centre for the wires from the battery box.
Feel free to download the file below for your own print. I have found these batteries on my Classic, SE/30, Centris 650, LC 475 and PowerMac G4 (Quicksilver) so they're quite popular in many Macintosh computers covering over a decade.