This is a peg grasper / peg receiver for ninja warrior training.

IMPORTANT. When creating ninja holds with 3D printing, the printables have natural weaknesses which run between each printing plane (i.e., the layers themselves can separate if precautions are not taken).

For this model, I do the following in order to maximize safety:

(1) I print at a minimum of 75% infill. 

(2) I use three holes for reinforcing screws or bolts, inserted at the bottom of the knob. These screws or bolts are intended to help prevent structural failure between the planes of the print where the load is primarily held (at the neck of the knob). I use 8-32, 2 ½” machine screws. See photos for illustration.

I then use 2 ½" #8 cabinet screws to attach the grasper to solid wood backing. (i.e., wafer head screws)

A spritz of textured paint on the outside of the grasper will give it a more rock-like finish and feel.

These graspers work great for me (I'm 6'1", 200lbs), BUT print and use at your own risk. 

Your safety is your responsibility.