The frame for my home security gate is made from steel box section and has large holes which allowed the frame to be mounted to the wall. The original plastic plugs (White plugs in the photographs) do not fit well and have fallen out and got lost. This is a simple replacement snap fit plug. I designed the snap fit to be tight enough not fall out while no so tight that they break when fitted. The orange is simply when I was testing the replacements, you'll (probably) want to print them the matching colour!

This design has an outer cap diameter of 34 mm, a cap height of 3 mm, designed for a hole diameter of 27 mm and insertion through a plate of no more than 5 mm thick.