[Caddylackn]

I would get one of those Knack construction site job boxes and bolt it to the bed.
It would protrude some above the bed but I don't think that is a bad thing. If somebody tries to break it open it will be obvious what they are doing to anyone that passes. These type of job boxes do a good job of hiding the lock so it can't be cut.

I would also consider adding a car alarm with a motion detention sensor, but turning down the sensitivity so it would only go off is somebody is in the bed or rocking the truck trying to pry this open.

No type of container is safe anymore, now that the thieves carry battery powered grinders with cutting wheels. What they don't want to do is be there 10 - 20 minutes trying to cut something open with a loud tool. If they can't cut it open in a minute, they will probably move on to the next mark. These job boxes are pretty hard and time consuming to cut open