I'm deeply tempted to fill up the rest empty slot, but I've been looking for the 2nd EDRAM module for years without luck.
I mentioned previously that I hadn't been able to find any EDRAM SIMMs... but that's not entirely true. It's more that I haven't been able to find any for a price I've been willing to pay, and frankly, that board isn't a high-priority project, so my price threshold is pretty low.
Here's the thing: those modules actually are available on eBay fairly frequently, but there's a bit of a catch. It's a little-known fact that some versions of the DEC KZPAC RAID controllers used 8MB EDRAM SIMMs as cache modules. Those RAID controllers are fairly common, the ones with EDRAM somewhat less so but can still be found if you look for 'em (most came with regular FPM, and it's usually not mentioned in the descriptions, so you have to either look closely at the pictures, or ask the seller to tell you what the chip markings say).
The problem is, of course, that you have to buy the RAID controller to get the SIMM, and between the purchase price and the shipping for a full-length card, it ends up being pretty expensive for 8MB of RAM. Though if you wanted to play with a SCSI RAID controller anyway, the investment might not be such a problem... you can pop a regular 60ns SIMM back into the thing and use it that way.