Yes FM, please use different (better) plastic for the fenders. I assume you can even use the same mold. Dirt bike fender type of plastic would work great! While you're at it, how about color choices for the fender?
It would be beautiful if FM made a swappable battery. Several fellow riders have taken their boards apart and the battery just slides into the channel. Woulda been great if it could have been designed so it could be removed / replaced without taking the entire unit apart.
You can do it with a solar charge controller. Normally you would set the controller to output 58v 3.5 like the wall charger. If you want to charge the OW less, set it to something like 56v and it should stop charging before the OW battery is full. For a source you can use a battery as in the Carvepower portable DC charger-
Rather than a battery, you could feed the controller with a 24v power supply that will provide 9A or more like this one . That will provide enough watts for the controller to charge the OW at the same speed as the wall charger.