Could you get the PCB to fit a commercially available case like the tada68 alu?
Unfortunately that won't be possible
The one that is pictured here is actually in a TADA68 case, but it's heavily modified to make it work. The mount holes that are on a stock TADA68 case would never line up right. Additionally, this keyboard is going to be built by mounting the switch plate to the case, as opposed to mounting the PCB to the case.