Simple part that slides into the top of your Ergotron base/clamp, and fits e.g. the small notebook mount from the fantastic Multifunction Arm system.

It prints great in both PLA and PETG without any supports on my Bambu P1S.

I'm a little annoyed with how far back the laptop ends up, so I might make a version with the mount plate offset from the cylinder part. 

OnShape link if anyone wants to remix it.