GoPro action camera mount M5 bolt knob (FreeCAD parametric)

Default is for M5 bolts. I attached the FreeCAD source file in case you need to tweak tolerances or change its size. Go to SpreadSheet, the parameters to specify:

external_diameter 10.
hole_diameter 5.1.
bolt_head_hole 8.6 - measured from side to side of the head.
hidden_bolt_length =26.6 mm - it specifies how much of the bolt is hidden inside the tube, excluding the head height. The lower the value is, the shorter the bolt would be. 

All the parameters should be specified with the required tolerances.

In case of recompute issues revise Fillet001 (it may be incompatible with some settings). The shape of the knob can be changed in Sketch002.