Hi, sorry for necroposting but I am dealing with the same issue, except for a boat (but the hardware setup behind is very similar).
Flexible solar cells can hep to cover uneven surfaces, but the commercial grade (affordable) ones have very low efficiency, 16-18% max and 4.7W per cell. Complete autonomy is a real challenge relying just on solar power.
Anyway, in order to reduce losses as much as possible I will use a MPPT charger, there are many models available according to the expected energy production, the one I am using also features a USB output port.
For a plane I would go with the small 5A modules with an incorporated DC-DC buck converter.
Now I was wondering if I should power the FCU via it’s power module or via USB; in fact, as the power module will be connected to the load terminals of the MPPT balance charger, voltage readings will be pretty useless.