You need stock roof rails. They are $270 from the factory and worth every penny!
Eurocampers sells roof rail fasteners, but delivery to Canada is a pain, so I made my own using 5/16″, 1″ elevator bolts and with the sides ground off. You’ll need some nylon nuts and washers to match the bolts. Four crossbars equals 8 attachment points. Way cheaper than the fasteners from Mercedes ($$).
To avoid the rails resting on the van roof itself, we need to boost up the height using 2″ sections of the 1-5/8″ unistrut, herein known as the ‘feet’. I had some strut left over from the propane install, which was handy. Chop off 8 of these puppies and file down the pointy bits!
The crossbar is the next layer of this sandwich, channel facing the sky. Eye-to-eye, the roof rails are 52″ apart. Unistrut comes in 10′ (120″) lengths, cut two in half, leaving four 60″ sections. The surplus rail (8″) overhangs the left side of the van, outside the roof box, to mount the solar shower. Perfect! Spring nuts and 1/4″, 3/4″ long bolts with washers are used to secure these sections to the unistrut “feet”.