Have you thought about bolting a locking, hardshell case to the back wall? I could also see welding steel struts to the back of the cab wall and then drilling and tapping them. Drill a couple of holes through the case for mounting and you're good to go. You could just weld or bolt a couple of hooks to the back wall and just bungie cord it to the back wall. Easy in and easy out while cased.
Make your own and patent it!