I'm after a bit of help here.
I have a crappy mag mount that I use when laning but with a small base, it dosent stay on long especially under tree. Last time out I stuck it to the rear bumper (its a fook of scrapion one) which worked ok but to be honest, its a bit pants......
I had thought of a gutter mount but my mate never had much success.....
Also thought of a fook of mag mount..... but not so convinced on sticking it on the roof.
Also had thought of something like one of these mounted on the rear door and cabled in..... not sure if the rear door is a good enough ground plane....