Thanks, I am glad you find this helpful. My plan is to continue to evolve this project as we get more learning on how to reliably deploy the Boron.
There is not special circuitry required for the Boron to accept a solar panel. I use the Voltaic systems panels as I find them to be very durable and long service life is my main concern. I use the 3.5W / 6V panel and only had to add a large 400uF cap to the DC-In to help buffer temporary variations in solar intensity. @Rftop, @rwb and others have helped with the power configuration settings you put in software to get the most out of the panel.
The big issue with the Boron is that its Real time clock stops working when it goes into the deepest sleep mode. So, if you want to use that sleep mode or use the Enable pin to power down the device, you need an external real time clock. Note, this also gives you one other helpful tool, it enables you to remotely power cycle the device.
Glad you are finding this useful. Going to take another turn of the crank with the carrier board based on @rickkas7 latest App note on watchdog timers: