Glitch has posted details on his build of a RAM board for the Challenger III:
With a quantity of 495 prototype boards in hand, it was time to build up a RAM board for the Challenger III. My implementation uses a single 32K x 8 static RAM in DIP packaging, which is split up into eight 4K segments, each of which can be enabled or disabled individually. It’s also expandable to 64K and beyond due to a few design decisions.
Project info at Glitch Works homepage.