I bought a system One rack this spring. It was the one on my truck @ Westwater. While it is not custom per se, it can be customized. It will fit on most trucks, and I am pretty sure that they make them for smaller trucks. The side beams(the vertical ones) have two channels that accept T-nuts, for mounting anything you can imagine. I have seen bike racks, ski racks, oar mounts, tubing mounts, lumber, etc... mounted. The top bars also have T-nut channels for more mounting options, kayak, bike, ski, etc. Most of the racks will fit over a camper shell. Mine is for a full sized truck and probably weighs 100 pounds, likely less. I beleive it is rated for 600 pounds.
I have a friend that had one that fit over a camper shell and had an expanded metal piece covering the hole top for sleeping on (road trips, when you dont want to empty the back of the truck), and ski racks on the side to make it easier for shorted peple to load and load.
Chech it out,