I have the Engel 45 model and I have it mounted in the truck bed using the Engel transit lock mount. Works great and the fridge is super easy to lock and unlock. Totally secure and I've had it about four years. Not sure if the transit lock will fit newer model ARB fridges. I suggest you contact the guys at Expedition Exchange. They sell both types and could definitely tell you what is compatible and what is not. See
http://www.expeditionexchange.com/engel/ and
http://www.expeditionexchange.com/arb/.
Only downside to the transit lock is that it is a permanent mount, and once it's installed, the bed of your truck won't be perfectly flat. This may or may not be a problem for you, but I find it mildly inconvenient on those rare occassions when I'm hauling stuff in the bed without the fridge. You could make it removable if you used the right kind of fasteners to secure it, but if you just install it by screwing it to the bottom of the truck bed then it's not so easy to remove since the screws are secured to the composite bed. This could be remedied with some creative fastening solutions that would enable the mount to be removed and reinstalled at will. I've come up with a few ideas, but haven't gotten around to implementing them. Wouldn't be too hard to do with a little thought.
The roller slide mounts are sweet, but they are even bulkier and your truck bed will be even less flat if you use one of those. That could be dealt with by using the aforementioned creative fastening solutions that would allow you to easilly remove and reinstall the mount whenever you need to. The roller setup would definitely make it easier to get to the contents of your fridge, especially when the tailgate is down. If I had to do it all over again, I'd get one of those and come up with a way to easily remove it when I needed to. I still might do it some day. The roller setup would also probably fit either the ARB or the Engel, giving you maximum versatility.
Agree with the other commenter about carefully considering where you position the mount. Load all of your usual gear in the bed and then figure out where the fridge should go, taking into account ease of access, your power source, and air circulation for the fridge (important). A transit cover is definitely recommended. Protects the fridge, provides additional insulation, and keeps out the dust. Worth the $ to protect your $$$ fridge.
One thing for sure...you're going to love your fridge. Once you've had one, you'll never go back to an ice chest. These things are the greatest invention since toilets.
