Make a Solar Powered USB Charger
What is being requested is very well documented and widely available for DIY, and following Instructable links are examples of that.

In fact there are several questions on EE.stackexchange site, which are currently appearing on the right side panel, as I type this answer.


Instructable#1
Instructable#2
Instructable#3
Instructable#4


Check your favourite online/offline store and you'd find semi-flexible solar panels, in various dimensions. They claim to be flexible to 20-30 degrees, and could be quite fine for the messenger bag.

They key would be selection of a good solar panel -- since there can be some significant difference in energy efficiency between panels, i.e. the extent to which they can convert the sunlight into electrical energy.

Also note that, not all smartphones charge / charge-well with the standard USB charging, i.e. they make proprietary deviations from the standard, and can expect to be able to draw (much) more than 500mA @ 5V.

PS> I wanted to put this as a comment, but unfortunately, I still don't have enough reps to do anything like that. Surprisingly, I am allowed to post this as an answer.