I found this 20 watt solar panel with dimensions that will fit perfectly inside and outside of the case:

This wiring diagram that I made based on examples from alternative-energy-tutorials.com (link) shows how to increase the total wattage of solar output by wiring the panels together in parallel or series. 

Basically by changing how I wire the panels together, I can control the voltage and amperage of the solar output which will increase the overall wattage too. My current portable battery is only rated for 5 volts and 2 amps of input so I will need to change my power source. This is a good opportunity to increase the battery capacity of the system which will increase the amount of time it can be used, and also the power delivery options available to me. With this change I have decided to commit to configuring the system to allow charging of a laptop as well. Stay tuned for more updates!