Built from aluminum profiles and extrusion (1020), the clamps can be found online ("2.5 Inch Tube Clamp Mounting ..."). It makes the Discovery barely higher, which allows me to drive it into the garage. The extrusion profile is perfect to attach lights, antennae and other attachments easily by inserting sliding nuts into the extrusion profile. It sits very low, but I don't use the front sunroof anyway.

 

Wow! That’s really cool! And if you got the extended OEM roof rack kit, it could be full length.

 

I plan to build one in the same style for the rear attached to the gutters. It will allow max width and max load capacity.

 

So I have used it for over 1 year with a up to 100lbs load on it without any problems. In case you are interested, here is a CAD file.

https://www.thingiverse.com/thing:6149732/files

Bolts and threads is 6mm, 1/4" works too. These clamps fit perfectly for attachment to the '04 roof rails:

 

Nice work! I made something similar out of extruded aluminum (great stuff!) for my ‘99 as I wanted some roof rack space in front of my roof top tent. The problem is that pre-facelift models have those thin bars, which are not in and of themselves a problem but they are a very bizarre trapezoidal profile I had to attempt to copy and CNC machine out. It doesn’t look nearly as nice as yours!