What are you going to make it out of???
Aluminum panels or a spun aluminum type? Is so then all you need is some good flat white metal paint rated for outdoors.
An all steel parabolic dish for "C" band would be very heavy.
Wire mesh? (supported) If so then the same thing for the paint.
Re-enforced fiberglass? If so then a thick coating (or multiple coatings) of aluminum paint between the last layers would work, or an aluminum window screen layer would also work.
Here is some info for making a parabolic dish antenna.
Parabolic reflector - Wikipedia
And a link to how to make a polar mount for it.
DIY Horizon to Horizon Polar Mount