I used 1-3/4" Split two piece shaft collars from Fastenal. I work there, soooo they were really really cheap. Not sure you would want to go that route as they are not cheap. You could use exhaust clamps, or a hose clamp.
This is why I kept the antenna at 3', just a little above the roof line. Parking garage? Whats that? lol. I can't get into those