Those self-piercing screw caps on the 1 liter Tetra Pak containers can be really hard to twist off. Even after you have initially pierced the membrane they can be tricky to get a firm grip on. This knob presses onto the cap and gives you a much better grip for opening the cap. It is designed for 25mm diameter caps, but can probably be scaled for larger diameter caps.

I printed this in PLA, but probably most any filament would work. I recommend using a 0.4mm nozzle or smaller.