I am just learning Onshape now. I am thinking of something similar to the design that was mod'ed for the IR sensor, but with 2 short pillars with a M3 square nut slot and inward facing bolt holes. The support pillars would be spaced apart to match the top mounting holes in the BLtouch. The M3 bolt holes would go beyond the bottom of the nut slot slightly so that there is a little room for fine tuning the height adjustment. I am also thinking about whether it makes sense to use the springs supplied with the BLtouch, since you could use them as an extra protective measure to make sure that hitting the bed by mistake doesn't ram the probe to the point where it is damaged. That's as far as I have gotten at this point. What are your thoughts?