Hi all -
Not wanting to spend £20+ per module importing them from the US, I've designed a PCB for an NVRAM board for Williams Joust, Robotron, Sinistar, Splat, Bubbles, etc. (NOT Defender)
It uses the Dallas DS1220AD nonvolatile RAM (which have had a fair bit of bad press but are great to use, and will also work with the MK48Z02). It has a jumper to select 2 sets of high scores and settings, and pads to connect the additional data lines for Bubbles which uses 8-bit RAM. You'll also need a socket for the CPU board, socket for the NVRAM, PCB headers, a resistor, optional capacitor and a jumper. Takes about 15 minutes to assemble.
I've a load of PCB's left over so selling them for £2.70 per PCB (plus postage from UK).
You can get the Dallas module on ebay for less than £10 so you should have a fully working module for much less than bringing one in from the US.