Then there is the idea of using mirrors like this -
How to boost any solar panel output by 75%
I had a different spin on the idea just a bit ago however that would actually work, again with the added complication of a tracker: Move the mirrors instead.
Something like this with a diffuse reflecting surface to better scatter and prevent hot spots. If the reflectors track inline with the sun, there is no shadowing, and much of the light reflected off the surface is scattered back to the panels.