Well, the best C-band circular depolarisers are mechanical, They are reliant on the waveguide dimensions and employ filters that are either part of the casting or are tuned using additional gates that can be adjusted with a screw.
I have used similar at Ku band (OMTs and separate filters), though in the main to filter out unwanted uplink frequencies and harmonics from the incoming downlink.
To improve steerable antennas from cross polarisation the professional manufacturers of offset dishes always use the reflector offset first, keeping the depolarising and filtering at the optimum / perpendicular angle to the dish. With a prime focus reflector, skew is the only option and the mechanical waveguide remains the gatekeeper to keep noise down.
