That looks like you have a Valentine. Is there any power in the harness that has power with ignition on? I have this clamp on mount I got from amazon. Hope I can trim cover to use this I hate the suction cups falling off in the winter.
Yes, a Valentine, I've been using them since the early 90's and will probably upgrade my 2 units soon (the new ones filter all the lane-changing signals and such from new cars).
I like that mount, it would probably take some re-engineering to make it work on the Transit, but where there's a will, there's a way.

Maybe rather than clamping you could run screws through half of it into the cover and/or the structure underneath it.
Yes, one of the wires has ignition + power (pictured), I plan to tap into it with a fuse to handle the detector and the GPS/Bluetooth that will be mounted there too.