Unless your cable is going to be very long and you have issues with resistance and voltage drop any gauge will be fine for the small current.
However since you plan to use it outdoors I would make sure that the chosen cable is UV resistant and suitable for direct burial. The easiest source is your nearest home and garden store where you can find multi-conductor sprinkler cable.
I don't know to which extend UV light contributes to the photovoltaic process inside a solarcell but you should expect at least some reduction in output because plexiglass (depending on type) blocks some or all of the UV light.--
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