For radome the best material I know is GORE, please see the adjoining link:
http://www.gore.com/en_xx/products/fabrics/microwave/microwave_fabrics_radome_material.html
Regarding the horn antenna for C band; for the numbers you have given me, you need a horn antenna with around 43 to 45 dB gain (in the case of 8' dish diameter) or 40.5 to 42.5 dB gain (in the case of 6' dish diameter).
Both horn antenna requirements are impossible for just a horn, (well impossible not, but not practical). See the attached picture than ends in 30 dB gain horn antenna.
Your horn antenna would be around 4 metres long, perhaphs 3 metres with a very nice design but in such numbers. Regarding aperture size, the reduction comparing with a parabolic dish would be 10 to 15% in aperture diameter.
Holmdel horn antenna is just something similar to an offset reflector antenna configuration built in a block.
Corrugations can be made availlable to work for C and Ku bands, the main problem is the feeding of Ku band but there are several solutions, in any case corrugation depths would be around lambda/4 for C band but corrugation period must be much mor dense and around lambda/3 for Ku band, this means that for a C band corrugated horn of your requirements if 200 corrugations are needed if it must work at Ku band you need 600, and this would increase price and weight.