Hello world!
My heating pipes and valves are locked in a box with a special 3-sided key (see pic).
Of course, during the years I lost the key and when I needed it, I couldn't find it.

I've created a spare key with N-sides. Check the Customizer for all the options.

Attached STL was created with following variables:
outer_diameter=12;
inner_diameter=11;
height_of_shank=20;
number_of_center_sides=3;
depth_of_center_hole=8;

I you don't like the design, feel free to download attached SCAD file and modify it.
If you like it, please click "Like" button:)