Old thread, I know. But have you tried looking for one in a junk yard? I picked one up for $30. Guy didn't have keys to it so he let me keep everything inside. (He told me to bring back any/all snap-on tools found tho....which I did once I drilled the locks...one 3/8 wrench lol) The diamond plate tool box sits on top of the rails for the tonnaeu cover which I cut a BIG whole in to fit the box. Yes it leaks when it rains but very functional, and I like how it looks. Not sure what your year would look like on top of the rails, but for an '89 I can see over it just fine in the mirror.
Different toolbox in this pic, but you can get the picture
*edit* Tried to link pic from my facebook page, decided to upload to an album here on RF to make sure link works*