I realized the current limited DC/DC converted I had intended to connect the solar cells to wouldn't work well (the input voltage was too low, and it might pull the solar cell voltage too low for it to be efficient.
However, the board seems to be out of stock at all English language sites in Europe, and not available on E-bay, so I decided to make my own version. It's available as a fork of Adafruit's GIthub project.
I added a zener diode on the input to protect against overvoltage from the solar panel (over 7V fries the IC), changed the input voltage reversal protection diode to a Mosfet, changed the connectors a bit, and improved the routing.
I ordered it today from Dirty PCBs (first try, before I've always used Seeed Fusion).