John, I will just speak from my experience. I am quite willing to believe that there are smarter ways to do what I’m doing.
My bean-drilling tractor was an old MF 2745 with a steel roof. I just slapped little magnetic antennas on the roof and it worked alright.
My next tractor project was an MT765 with a plastic roof. I plan to mount a bunch of other antennas and cameras so I built a big frame for some much better antennas on steel plates. It worked well. https://scontent.xx.fbcdn.net/v/t1.0-9/13606856_10106613732628728_9004130183656332863_n.jpg?oh=957db26703747866d494fc5118e180f9&oe=59427BB8
For a stick-on antenna, I’d definitely use a good steel surface for mounting. The specifics will vary by tractor. Do your tractors have existing mounting points? (My Deere equipment has mounting plates.)
Working near shade, I’d feel a lot more confident going with a bigger antenna. The larger (surveying) antennas often mount on 1" NPT. Mounting one of those is just a matter of securing a regular pipe flange base and screwing in a pipe nipple. Moving the antenna (and nipple) between tractors should be fairly easy.
Good luck!
–kyler
Also, I realize you’re using RTK, but consider putting the antenna as far forward as is reasonable in order to increase accuracy. A Simple Method to Improve Autonomous GPS Positioning for Tractors - PMC