The internal diameter starts at 20.54mm and decreases to 20.09mm. This allows for a nice press fit onto the end of the rod that you are attaching it to. The end knob was made with large fillets to eliminate sharp edges and lower the risk of snagging.

I made this for a cheap Amazon 2-Tier Bamboo Clothing Rack so that I can make use of the outside post without hangers getting bumped off.