This is a small sliding door security arm stopper, which fits into the end of a security arm. I lost the small rubber tip to the security arm on my generic sliding glass door. I couldn't find a cheap replacement, so I made this quick design to take its place and keep the security arm locked against the sliding door. 

Printing it in PETG produced a product that can withstand some compression when locked into place. 

The part of this stopper that inserts into the security arm is 15.9 mm x 15.9 mm, although the model is relatively simple and can be scaled up or down as needed.