I'm periodically polling this topic for news. How is it going with your keyboard?
Development is on hold for the moment, I'm afraid. Lots of "real life" keeping me busy, so not much project time. The keyboard is in daily use on my home PC and does the job very well. I do still plan to build a "Mark II" version (either from curved acrylic or with flat steel plates like the first one and wooden cases) with a controller in each side, but it has to wait a little until I get more time for keyboard projects.
The reason for having a controller in each side is that either can be used on its own as a game controller or one-handed keyboard. At startup the firmware checks if the USB bus has been configured (that there is a host present) and if not, goes into I2C (master) mode. If there is a USB host it goes into "combined" mode (I2C slave mode + scanning local key matrix + handling USB). So either side can be connected to the PC and then simply linked to the other via 4-core cable (RJ11).
I think steel is the best plate material, although aluminium could work for plates this size if it's the right alloy and acrylic / perspex works well enough, too, but the thicknes requires the switches to be glued in place.