I did similar, but used two of these (from a pack of six). One in the top box and another under the seat. They automatically turn on when it's dark and there is movement in front of them. Can also be turned on manually. USB rechargeable, not needed so far. The plan was to use USB powerbank, plug it in when starting journey.
I fastened it using glue on velcro so it's easy to bring it inside to charge. .