Will these help? Have seen them at West Marine. This photo is from Fishmater T-tops web site. I know some of the t top makers around here use them in the canvas tops to make holes for fish rods.
I go through the top on the frame plate the the antenna mounts to. I use a Blue Sea Cable Clamp. Expensive but virtualy waterproof! To avoid tearing the canvas, I use a hole punch of appropriate size and the clamp seals everything nice/nice.