Where are you wanting to mount the antenna? What type of mount, mag mount, stake pocket? I've already drilled a 3/4" hole for my GMRS mount and will be turning the mount for my CB antenna soon. That will also be a 3/4" hole through the roof. I've had that setup before with no leak issues. You just need to replace the sealing o-ring every 5 years or so depending on conditions. Right now, my Cb antenna is on a mount to front fender with cable routed through clutch cylinder plate. Is your pre-made cable have the smaller connector that screws into the larger connector?
Of course the best place is on the roof for the ground plane provided. I went with fender mount for speed in getting it mounted for use. Here to help with questions.