Do the solar panels extend past the width of the Yakima tracks? If not, then below can work. If they do, then the angle brackets will need to run laterally.
1.5" x 1.5 " or1.5" x 2", or 2" x 2" Aluminum angle, 1/16" or 1/8" thick does the job. The horizontal dimension depends on the spacing width of the Yakima tracks and width of you solar panels. The vertical dimension needs to hold the panels high enough to provide for airflow under the panel, minimum 1" below the lower edge of the panel. You will need to get the T-Nuts* to go into the Yakima Tracks. Best place I found was Grainger. You will have to get the right depth button head screw (allen head is preferred) to allow for the thickness of the angle, washers and threads in the T-nut. Proper depth is necessary so as to not bottom out on the track. Use some blue Loctite on the threads.
* If the track end caps are removable, you can use a square flat nut in place of the T-nut.
Mounting to the solar panel requires care in placement of the mounting holes. Make sure the holes are low enough so as to not interfere with wiring or the glass, and provide room for washer and nut to clear.
Lastly, The roofs are typically not flat, so two long brackets on each side (if the panel fits inside of the tracks, are preferred to one long one.
I'll see if I can get some pics of what I did with our panel. PM your ph # to me if you would like to discuss.
Beastie 3: 2002 7.3 EB Cargo: Agile TTB, CCV Mid Top, Custom Walk Through, Lots of stuff added. BlingMyRig.com