I fitted these at the weekend c channels on both sides of my van - I modified them so as to not interfere with the “leak free fitting method” of the roof rack. The modifications consisted of shaping the front of each C channel and drilling 4 holes in each channel (3x 20mm to fit over bonded washers and 1x25mm to fit over VW original roof bolt) I also stuck them down with double sided automotive tape. Then replaced the rails on top. They will be used with “The Tarp” (stitches and steel) this means I can choose to put it on one of three sides of the van - left or right c channel or over the tailgate (not yet had chance to figure out how to attach it in the tailgate position. By the way the “the tarp” kador strip was a very tight fit into each channel.
looks good - can I ask where you bought the c-channel? Is that 6mm?