With C Brackets being so expensive I had my own cut out of 6mm Aluminium on a water jet cutter. Very pleased with the results and the flexibility of the solution.
I'm now also pondering and "O" bracket to use at the front (instead of one of the C Brackets) as this will provide added protection.
The base plates are to accommodate the Manfrotto quick release plates as these need clearance underneath and I extended to top one to receive the foot on my 70-200 and 100-400 lenses.
It is certainly proving a robust solution and the multiple quick releases mean, at the flick of a lever, I can remove the rig from the sticks to run 'n' gun or just mount the camera without the rig, or use a monopod which has the same release (with or without rig) all without having to swap mounting plates. Result. :-)