To enable moving our clothes drying rack on castors, I created adapters for the castors.

The castors have a base plate of 40x40mm and M4x25 pan head screws are used for attachment (in my case, the head of a countersunk screw was too large for the castor). The M4 nuts are countersunk in the model.

Inexpensive clothes drying racks often come with a hole for clipping-in plastic feet. The adapter uses this hole for secure fastening.