I've recently installed a lithium watch style battery onto the board of my A2000. Cost me only a few bucks for the parts ;-)
You'll need a cr2032 or similar battery, battery holder and a diode. There's instructions on Aminet.
I broke the little legs of the Amiga barrel battery and soldered them onto the new battery holder. This enabled me to solder the battery holder directly onto the motherboard rather than mounting it elsewhere.
