You are obviously thinking of heading to the great outdoors .. the waterlily is okay if you happen to have a river or a stream closeby. If you tried using one round here , it would be gone when you went to retreive it !
A couple of years ago I was given a small Silvercrest solar panel for charging USB devices. It is not a lot bigger than a mobile phone .. It came from Lidl and works perfectly. You just sit it on your dash and it trickle charges the device fairly quickly if the sunlight is strong. Have no need for it now as my new phone charges from the mains in 20 mins and lasts for 3 days.
Have a look here .. there are plenty to choose from.
https://tinyurl.com/y555fjq5