I have successfully used 12Volt / 10 Watt and 12Volt / 20Watt solar panels to charge and run XOs.
Here are some typical examples of panels on eBay:
The 10Watt panels will provide 0.5Amp in bright sun, and the 20W panels will provide 1Amp current.
The XO uses around 1Amp in normal use.
So both types will charge the battery if the XO is not in use, and the 20W panel will run the XO without depleting the battery.
The 20W panel will charge the XO from flat in around 3 - 4 hours, and the 10W panel will take twice as long.
Often the battery is only partly discharged after class use, so the time to top up is obviously less.
Of course the 20W panels are proportionately bigger and cost more that 10W panels, so you have to decide what is appropriate for your usage.
Note also that some panels are sold with a charge regulator included, which you don't need as there is one built in to the XO.
With regard to the connector, it is very unlikely that you will find a panel supplied with the XO type connector.
I think what you will need to do is purchase panels (with leads), and purchase connectors separately, then attach the connectors somewhere in the delivery chain.
Example of the connectors:
Note: The connector on the XO is specified as having a 1.65mm centre pin, which is not a commonly available size.
Mains chargers used with XOs commonly have a socket designed for a 2.1mm centre pin, which is a more readily available size.
It would pay to evaluate candidate connectors before purchasing in large quantities.
Hope this helps.