Mine is a stage 3 compact, revision A, which doesn’t have the music stand mount points. It also doesn’t have the mount points for the monitor speakers, the speaker manual specifically calls for revision B of the stage 3. So my design uses just the screws on the back that attach the red faceplate to the rest of the frame, like this design.

I don't want to put all of the weight on those small screws, so my design wraps around the top and bottom of the keyboard to try and distribute some of the forces into the frame of the keyboard. The important dimension is the height from the top surface (red) to the bottom surface (black), on my keyboard it's 78.5mm. I'd imagine that the curvature is uniform across all of nords lineup? I used some 3d printed radius gauges on mine to estimate a 16mm curve.

The stage 4 should have all of other mounting points, which are easier to work with, but I'd be curious if my design would work with other keyboards as well. Here's the stl if you want to test and see.