These are replacement pads for quick-grip tools that allow you to use your clamp to hold two pieces of material at a 90 degree angle.

These pads are designed to fit standard quick-grip clamps from the Irwin brand, but may fit others.

One side of the clamp interface is open, allowing you to slide the grip into place. Static friction should keep the pad in place, dependent on your printer tolerances. If the pad slips or is difficult to slide on, try heating the pad gently with a heat gun or adjusting the size of the print to suit your printer's tolerances.