A) materials: J channel from the electrical section of your local hardware store. One ten foot section was less than $20. Thank you to the other author who suggested this, it is fairly strong, cheap, not too hard to work, and has holes to make attaching easier. I cut the j channel with a metal blade on a table saw and used a file to smooth the cut.
Four u bolts - Make certain they are big enough to go around the roof rail and j channel. The J channel can be flipped to allow for a longer or shorter bolt. If possible make sure they will fit through existing j channel holes but a jigsaw with metal blade will enlarge the j channel holes if needed.
Pipe insulation about two feet to cut to length for padding.
Four zip ties to attach insulation to j channel
eight wing nuts to match u bolts if you prefer to be able attach and detach the rack tool free
can cut eight spacers from pvc if needed (if the threads are above the j channel once it is laid on top of the rails)
B) Preparation: measure and cut the J channel to the desired length. Make sure they are at least long enough for the u bolts to grip on both sides. I made sure mine were short enough to avoid head banging as passengers get in and out. If you ever need a longer rail temporarily you could probably run/attach a longer tube or 1 by to the j channel.
Check the fit and then cut the pipe insulation and attach with zip ties so your roof and roof rails will be protected.
C) Install: Lay the finished j channel where you want it attached on the roof and ensure the U bolts fit. The j channel holes can be enlarged with metal cutting blade on a jig saw if needed.
Remember you can attach the j channel like a "u" or like a "n" as you wish. I wanted the U because I may eventually shorten the bolts so they do not stick out above the top of the rail (may have to make my own bolts from threaded rod).
Make sure everything is tight. Use washers if needed.
Total cost about $30