This is a rocket mount for a Mobius action-cam. This has been designed for an eight inch diameter rocket.
The holes are for 3 mm wood screws.
You can easily modify the mount for any diameter rocket you need by editing my tinkercad model here:
https://tinkercad.com/things/4f7c9ilTxNp
All you have to do is change the diameter of the cylinder "hole" that is cutting through the mount, and realign it to the mount when you are done.

I added a second mount, this one with a shrowd that blocks the wind from the upper section of the camera - very useful if you need the aerodynamics, or you want usable sound from the actioncam. The STL is for a 75mm rocket, but I've added a second file in tinkercad that will allow you to use this for any diameter rocket.

The copyright on this is unlimited, non-commercial. I know someone who sells a similar (but superior) shrowd here: https://www.etsy.com/listing/177005125/mobius-hd-camera-shroud-for-rockets-3d, and I have no interest in hurting his business.