Free
It's free to download
Technical
To screw the left-right-mount parts together:
2 pcs M6x30 mm bolts
2 pcs M6 nuts
For the long GoPro knob:
1 pc M5x50 mm bolts
If you want metal threads instead of printed plastic:
2 pcs 1/4-20 UNC nuts
2 pcs 3/8-16 UNC nuts
Free