[Apex]

How about the two sets of holes at the tailgate end of the bed sides? They likely have a small press-fit plug that could be removed to expose the tapped holes. I'm not sure exactly on the pitch of these threads (anyone care to share?) but you could make a bracket to work with those and run a locked cable from that to your cooler or whatever else you want to lock down. I think there even is an OEM bracket out there than can be found on ebay for less than $20 but custom may suit your needs a bit better.