A knuckle duster that was designed to be as ergonomic and comfortable as plastic can be.
For durability I recommend printing at 100% infill.
I recommend printing on a smooth plate for adhesion.
I left a large smooth surface behind the protruding segment to allow for soft foam to be glued on for added comfort.

(IMPORTANT) Sizing Guide:
Measure all your fingers, from pointer to pinky, take the largest size you get and resize the model to fit it.
The model by default is sized for US 11 or 20.6mm diameter fingers.