Disk magnets are cheap and a guy could turn up a "base" on a lathe pretty quickly.
Question I have for you Gene is have you tried the SwiftNav receiver yet? I am curious if it works. Those used 500's on RTK would be a good fit for our little Versy 835.
Haven't yet.
In case you haven't had the EZ Steer on your 835 yet......on an articulating tractor mount the antenna as far forward on the front end of the hood as possible. Roof of the cab doesn't work very well because if you turn the steering wheel one direction the cab goes the other.