I stumbled across the concept of Ambiguous Cylinders at the makeanything YT channel. These were shown by Kokichi Sugihara in Japan.

I modelled a circle/triangle variant myself and arranged them to act as an intriguing pen holder for my work desk

For printing I suggest using a 0.6mm nozzle to save some time.

If you want one and do not have a printer or the filament color you like, DM me.

You can find this Pen Holder on my Etsy Shop