I've designed the method of mounting everything to keyboard cases. The idea is that pressing down on the belt area will slightly flex the PETG L shape and a strain sensor circuit adhered onto the vertical can be used to detect force. The verticals are under 7mm thick so likely would still flex a detectable amount. M2 screws and thin nuts are used.

I haven't yet determined the way I want to mount the 2 hall effect sensors at 90 degrees.

The UHMWPE tape is also now visible in the model. The belt length is currently 92.5mm.