I'm using one of those at my masthead, 75' above the waterline of a 54' sailboat.
I was also concerned about the mounting of it, but I think I worked it out ok. I bought a Shakespeare stainless steel
mount which has three 8mm bolt holes in it. Then I had a stainless steel
plate made to fit over the masthead truck. I bolted the plate on with four 8mm stainless bolts with lockwashers and loktite. I electrically isolated it with that flourescent green paste stuff, forgot the name of it. Then I bolted the Shakespeare mount with three more 8mm bolts. It seems very robust and I am hopeful that it won't shake loose. I have done 1000 miles on the boat like that and so far no problems.
The improvement of an antenna like that over a whip is incredible. I would not go back to a whip, myself. There are other threads on that on here, if you do a search.