A cheap and convenient replacement for the PRAM battery is a standard battery holder, like for AA or AAA size cells. There are some threads regarding the PRAM batteries for desktop Macs. PowerBooks have similar requirements, only less space available inside the case. Just check for the voltage you need and build a battery pack with approximately the neccessary voltage. Keep the original PRAM battery plug and extend the wire to desired length. In a PB1400 I found some space in the HDD compartment after replacing the HDD by a small CF-IDE adapter. Else you could stuff the PRAM battery into an unused expansion bay or hide it inside a dead main battery case. Make sure to wrap the battery to avoid damage from a leaking battery.