A simple pole clamp for multiboard I made using the Dual Snap Connector “Standard” fitment. I was mounting my multiboard free standing below a desk shelf and needed to mount the sides to the poles holding up the shelf. Unfortunately these are a bit specific to my setup, being a 30mm pole and 20mm pole, with certain distances. But these should be easy enough to remix for your own needs. 

Alternatively If you have this VIVO Desk Shelf https://www.amazon.com/VIVO-Workstation-Accessory-Organizer-DESK-SHELF55A/dp/B0CDF7HXCY/ref=sr_1_10?sr=8-10 then you can use these to mount a 3x6 multiboard grid :)

I used PETG for these but PLA+ would work as well. I wouldn't trust PLA but it'd probably work.