OK, I found the perfect fit tool box for my needs. It's an aluminum crossover box intended for a compact truck. 61" wide single lid, 49" wide lower portion, 20" deep front to back. I sit it on the bed up in the front, so the 49" wide part fits exactly between the wheel humps, no wiggle room at all (maybe even rhinoliner would prohibit a fit). I have it bolted to a bar that I ran across the front tie loops. I like it below the rails so it doesn't block any window, or make it higher to lift stuff tio load.

Pics:
http://pg.photos.yahoo.com/ph/mmiller2002/album?.dir=/9684
The closeup pic with the lid open you can see one of the bolts that hold the box to the crossbar I had there.


Got it at Northern Tool & Equipment, their label, made by Better Build, item number 348146