Morning "crit"
I have been doing the same, sorta. We have a fiberglass shell on our 99 Super Duty Crew Cab short bed that I made a custom rack for. I made it long enough for a 2.5 meter awning and it works well.
But I would like to put a flat rack on the front of the cab mounted to the roof of the cab and you are correct manufacturers do not make mean things specifically for full US trucks.
While at Overland this year I spent some time looking just at roof racks for this and the best I found were the Front Runner aluminum racks out of South Africa. They have a US branch in the US located in the Los Angles area of So Cal. What I like about them is they can custom make a rack close to a size you need and you can choose how many cross slats you want in it. You can have it a solid bed or with opening of almost any size.
The restrictions are based on the extruded aluminum slats as they only have one design to work with. I believe they are about 4" wide?
The Side rails, front and rear rails along with the cross slats all have slots in them for hex head bolts and tie down loops some company's are making now.
They have different style of "universal" mounts to choose from, but basically we will have to drill the roofs and put in nut inserts to mount they to our roofs.
Hope this helps. Daryl