SheepsFeaTheR made a fully 3D printed cap opener and when looking at the design the use of a 6mm GT2 belt came to mind. I guess that most 3D printer owners will have some GT2 belt lying around while maybe not everybody has TPU or TPE filament for printing the belt.
Hence I remixed his design to make it work with a standard GT2 belt. There are two versions - you can use what is best for your application:
I 'borrowed' Prusa's design from the X-Carriage to keep the GT2 belt in place.
Credits to the original designers.
Using this 'tool' is quite easy: wrap the belt around the cap, tighten it a bit and slide in into the groove, push the belt down at the back end of the opener. The belt will not slip in the tool.