This is a clamp I use to hold the lamps in my living room.

The center hole is 7mm wide and can be clamped to 5mm.

The screw is a m3 screw with 5.5mm nut and head (the clamp has an hexagonal hole to fit the nut). The length of the screw I use is 16mm, but you can use a longer or shorter one.

The outer ring has a diameter of 25mm. The height is 7mm.

I printed it in PLA, but probably better in PETG, as it will bend more easily.

Also, I recommend using the "Archimedean chords" infill pattern to make it more flexible.