There's not a whole lot you can do to improve things with RF - it's the lowest quality signal type supported by most retro consoles, and CRTs that only had that as an input were generally older or budget sets.

Make sure you get good quality coaxial cable that isn't fraying or super old, and keep daisy chaining of adapters to a minimum. Also, try to get RF Modulators that are OEM whenever possible. It's still going to look rough compared to other signals, but that's all you can really do.

By the way, there's a CRT magazine on Kbin if you or anyone else would like to join (questions like this are encouraged over there!)