You might have to make your own - I think they went the way of the 'do it yourself sun roof' (which went away because a lot of times they leaked if they were not done just right).

Check to see if the roof of your cab is double walled. If it's only one layer of steel, you cant run screws into it, so making an overhead shelf would call for some kind of ribs to keep it from sagging.

If it's doubled, you can use anchor bolts or PK screws to help support it. IF there is enough space between layers.