I really don't think you'll have much success trying to seal coax!
Rather, I'd use a 'bulkhead' fitting. Such things are available for the N- and C-connectors used in commercial applications where the feeders/antennas are pressurised with dry Nitrogen or they're feedthroughs to vacuum-equipment: I specced such a beastie for a HF ATU I designed a while back.
Not cheap though! See
https://www.fischerconnectors.com/uk...cations/vacuum