I don't think modifying a GH60 will work. I wasn't much involved with the project (and still don't have one myself) but as I recall, most of the controller was under the spacebar. You'd likely end up needing to drill holes in the microcontroller to get a switch to fit there, but I could be wrong.
To make a plate, you'll need to add switch holes, stab holes (I recommend costar: very easy to make plate holes for that: they're just rectangles) and a border (for standoffs or to bend down like the upcoming massdrop stuff, ron's GH75, or the original dox prototype). If you don't have a PCB, then just square holes for the switches is fine 'cause stem swapping likely won't happen, and it's easy to pop them out of the plate if you really need.
The only issue I see is (step 7) that mounting the plate to a case may be hard. You might want to have the edge of the plate "extend" to the edge of the case, (so it sits on top rather than "in" the case) and then add screws and/or washers to hold it in place to the mounting holes in the case. What was your plan on this?
I must say, I really like the bottom row layout you have here.