There is/was a company in Denver that made cases for AR-type rifles. They were widely sold at local gun shows for reasonable money - less than $60 for sure. I think it's a pretty decently shaped/configured case and a great idea. I'd also pick black over camo, as camo sends a message if spotted. I would try to attach it such that when the gate is shut, the bag/case is resting on the floor of the truck so that the constant bouncing doesn't tear it from the gate.
Steve H. - KE0KNH
Colo Springs, CO
'78 FJ40, '89 FJ62, '98 4Runner