Do you usually use an umbrella when it’s sunny or would you do it just to charge the battery? I know it’s a thing but only very rarely see people do that, probably not enough to have demand for something like this. Backpacks with solar panels exist but I think these are pretty hard to find too.

And you can continue the fun when they start experimenting with it and inevitably fall into option 1.5 and have the window hanging from one corner.