I have a NicroVent in the head
and I leave the head
door open. The vent in the dropboard completes the ventilation circuit.
The solar panels
need to be regulated to run the fans. While you can get away with these sort of big swings in voltage with LEDs (8-30 VDC tolerant, if I remember correctly), something with a motor
in it will be damaged or will melt if you ramp
up the voltage to nearly twice what it's rated.
The way to do this is to regulate the power going into a fully charged battery
to "float" range, and to run fans from a fused circuit.
If you don't want the fans running at night (which would be the case with solar, even regulated solar), run a 12 V timer inline for 8 PM-8 AM shutoff.
Really, it's easier to just keep your batteries charged via solar to a voltage set point, and to install a self-contained solar powered vent.