The photos show a rack we are building at the office as part of a research project. The material is from MiniTec T-Slotted Aluminum Extrusions. Modular Aluminum Profiles For Custom Construction From Aluminum Extrusions. Custom Clean Rooms, Ergonomic Workstations, Belt Conveyors, and Machine Guards Are But A Few Applications.
and as you can see in the photos it is quite substantial aluminum. They make various sizes of this extruded stock, with slots that allow mounts and adjustable tie downs. Prices are not bargain basement, depends on size of stock. From my short time looking around the forum it appears Rover owners value a good rack.

 

A good roof rack is a must on any Disco.

 

Spike, I wouldn't recommend sleeping up there.

 

Man, what is that, 4" square?
That would be so overkill for a Land Rover it's not funny.

 

These parts are about 4 inch square, but the supplier makes the extrusions in smaller sizes.

 

I've seen this type of material used before to assemble fixtures for optical instrumentation.

For a general purpose roof rack on the Discovery it is not an appropriate material because it is too bulky with needless rigidity. The load bearing capacity of the Discovery's roof is too little to make any use of it, especially if you are just mounting on the gutters. It could be good if you were using a single piece of it to mount a single piece of equipment or instrument.

A similar material is simply t-slotted extruded aluminum flats. Find my earlier post on roof racks and see the photos of the Hannibal Safari roof rack that uses these t-slotted slats. They are essentially the same but with only one t-slot surface instead of four.