VHF antenna bracket

This is part of the initial design of a bracket to hold a VHF antenna. The bracket will be rail mounted, the diameter of the rail being 19mm. The bracket is in two parts. Other parts are required to complete the unit, these were sourced from an existing mount.
A picture of the existing style of mount is included.
On fitting the unit other improvements have been noted:-
The holes on the lower section could be changed to hex shape to accommodate nuts without them turning. At the time of design the cap screw orientation hadn't been decided.
A further skim could be taken from the mating faces, 0.5mm had already been removed, I would take off another 0.2 to 0.5mm from each face to see if clamping force increases, although it is adequate as it is.
Another modification that could be attempted is the reduction in diameter of the through holes, these are currently 4mm. It was designed with 4mm S.S. cap screws in mind, 25mm long. If one half of the model has the hole diameter reduced