I hear you, people in this world have sticky fingers and will steal anything and everything. I came up with a few ideas, not sure if they will work.. Just trying to help you brain storm...
Idea #1: A small little 'store display like' stand that I found on amazon, not sure how secure it will be, however i'm sure you can modify it just a little to make it work. Take a look at the links below:
[Aftermarket Product] Brand New Black Anti-Theft Security Telescopic Cell Mobile Phone Display Stand Holder Unit
[Aftermarket Product] Brand New Anti-Theft Security Alarm Charging Display Stand Holder For Mobile Phone Digital Camera
Idea #2: Drill a hole through the side of a case, put some sort of screw-loop with a flush screw head on the inside so the phone still fits inside of the case. Then when there is a loop on the outside of your case you can run a cable like the link I have below through the loop and maybe secure it to your cab. Maybe be a little 'hokey' but i'm sure it would work.
Buggyguard Anti-Theft Retractable Stroller Lock
Idea #3: A secure bag, I'm not sure if this would work because i'm assuming you need to see the screen, but it's a suggestion. A secure bag that is secured to your cab.
Travelon Anti-Theft Lock-Down Pouch