I lost the original clip, so borrowed one from a friend and made my own version. You will need an M4 20mm machine screw and bolt. I printed this with a raft and supports at 0.2mm because I was in a hurry.
I have just discovered that when cold, the clip part is prone to snap, obvious really. A tough, flexible or nylon filament will help, as will printing with thicker walls or more infill. Perhaps rotating the clip through 90° so the part lies lengthways.
The BikeLightCollar.stl file has an internal diameter of 31.6mm, which is the dimension of the original part.
The BikeLightCollar27mm.stl file is a version that better fits the seat stem on my bike at 27mm diameter.