At the moment, I think a simple rolling panel would be ideal for most of the vertically stacked common style indoor farms. This would allow the panels to be moved to the most advantageous location - probably near the youngest plants - to collect the most unused energy.

For my own test, though, I would really like to set up an enclosed plant cabinet with solar panels in the non-glass sides to collect any spillover lighting. Using the energy generated to power data logging sensors to track plant growth and conditions would be an ideal use for my situation, my test so far should be able to easily power a low usage scenario like that.