This is a 1u tall version of my tunnel baseplate; at this height, the X/Y tunnels go away, leaving just Z oriented 3u (21mm) rounded holes.

Baseplate sections are joinable with rabbit clips, and have pockets on the bottom for stick-on rubber feet (defaults designed for 6x2mm rubber feet).

Slicing guidance:

• import an STL into your slicer
• split the clip object from the baseplate
• slice the clip with no top/bottom layers, no infill, and 2-3 walls ; they should be only wall paths
• duplicate the clip as many times as desired

Parameterized openscad model for easy customization: https://github.com/jcorbin/gridfinity-risers