I haven't done anything exactly like this, but it sure sounds like a real interesting project!
Hope you'll post pictures- I'd like to watch the way it evolves...
Is this going to be a permanent mount, or will you need to take off/put on?
If it's permanent, can you bolt it to the bed using longer versions of the same bolts now holding the bed itself on?
Another (additional?) point of attachment that occurs to me is bolts through the utility boxes' inside back walls, presuming that the boxes are securely attached to the bed...
Keep us posted!