I talked with Mark at http://www.metaltechcage.com/ about building one. He said he could do it but he would need an 80 to make the bends and it would take awhile to get it right. He uses good material and the welds were good on the cages I saw.
I've designed several in my head. Mostly with a hoop behind the front seats with an angled bar across the bottom. Then with sidebars going across the top by the Back seat grab handles and then a hoop behind the rear seat and finally one in the back. Doing anything for a very front hoop becomes difficult due to the dash and not wanting to lose visibility and foot room.
Folding seats, rear seat leg room, and actual amount of interior space make designing an efficient and safe cage a challenge.
Some of this has been discussed on other boards, and based on pictures from crashes/rollovers, it is quite apparent that the real need for a roll cage is to strengthen the A pillar location. Most of the photos I've seen show this area caving it and the rest of the truck looking quite good (especially those with an integrated cargo barrier after the second seats - my 'quasi roll bar').
The last post shows a cage that has this, but with the bar being very noticable.
For what it's worth, here are a couple pix of the cage in my racecar.
The cage is a bit overbuilt, as you can see. It was built by Russ Nyberg @ Racetech Fabrication in Portland Oregon. There are a few pix of "Version 1" of the cage in his gallery, as well as. He builds cages for just about anything, ground up tube chassis racecars, etc... If it's metal, he can build it. For a point of ref, my cage cost right around 1500 bucks. It's constructed of DOM steel tubing (1.5 inch / .095 wall) http://www.racetechfab.com/gallery.html
Not that going to a fab shop would be everyone's choice for an offroad cage. Just thought youz guys might like to see an example of a custom cage and cost reference. He's pretty competitive though, I had looked into several shops with estimates from 1200 to 3500 bucks to build this cage. If anyone's near the Portland area that was interested, IM me, and I can get you hooked up with a good deal thru Racetech.