Remember, the two factors controlling resistor selection are what is the highest RF voltage expected across it (under normal operation), and how much power might it dissipate (also under normal operation). You may want to consider going all the way, and include a GDT in the design. A few years back NY4Q had a short post on QRZ about lightning suppressor for open feedline that was for sale. I'm not able to find it now, but had a brief circuit design and Mouser part numbers. Other than the mechanical parts, it required two GDT devices, two HV caps, and two 1-meg 1-kV bleed resistors. Looked to be fairly simple to build. Some of the HV resistors were unobtanium during the last several years, but I have secured a few for my needs/projects.