been building a custom one for my '03 DC. Might give you a few ideas. I didn't want one over the cab for aerodynamic reasons and I also rarely pack things much higher than the top of the bed so i didn't need the tent up so high. I gave myself an extra 6" or so above the bed rail for gear underneath. Mine serves double duty as a place to mount a RTT on occasion, but also as a regular rack for dual layer gear storage and a place to carry bikes/snowboards etc and still have room underneath.
Sorry if there are too many photos, but these are through the build process.
The narrow gaps on either side of the rack are for my tiedown track system.
With the old style Mombasa mounted up. It's a little long for a DC, but it will work for now until I pick up an EeziAwn/Camping Lab/ARB tent in the future that won't hang off the back quite so much.
trying to get it all finished up in time for a long trip over Labor Day weekend.