The sprinter is already so tall, I really don’t want to add any storage space on the top. However, I would like to install a solar panel and mounting points for an awning, or tarp hook points.
The 2006 and earlier sprinters did not come with roof rack rails. I believe the 2007 and newer models have rails, or option for rails.
My initial idea for rails involved using 80/20 or similar T-slot extruded aluminum. These are stiff, very strong, and the position can be adjusted anywhere along the length.
Later, I found out about L-Track, used for tie downs, and similar to what they use for seat tracks in airplanes. The track is lower profile, has nice form fitting profile, and the attachments can be inserted anywhere along the track. There are a lot of options for tiedowns that insert into the track. Most are quick-release, but some are more permanent screw down.
An example of L-track available from Mac’s Custom Tiedowns (Summit Racing is a local distributor of their products):

End pieces are available here: http://www.macscustomtiedowns.com/product/257/All

- Suppliers of L-Track
Sealing up the underside of the track:
Hardware:
- Screws
- Nuts
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90572A400 18-8 Stainless Steel Tab-Base Weld Nut 1/4″-20 Thrd Sz, 3/4″ L X 1/2″ W Base, 5/16″ Barrel H In stock at $10.59 per Pack
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- Sealing Washers:

