A 7.4 volt lithium ion battery having 860 mAh (my Canon 2000D uses this kind of battery) has 6.36 watt hours of energy. Even after the various voltage conversion and charging losses, charging it won't take more than 10 watts if done in an hour, or 5 watts if done in two hours.
Sun provides 1000 watts / square meter, of which a solar cell can capture perhaps 16% or 160 watts per square meter. For 10 watts, sqrt(10/160) = 0.25, so for 10 watts, 25 cm x 25 cm solar panel will be enough if its normal is facing the sun. At an unoptimal angle, let's say 35 cm x 35 cm will be enough.
For 5 watts, it will be about 18 cm x 18 cm at an optimal angle, or let's say 25 cm x 25 cm at an unoptimal angle.
For my tastes, these chargers are way too large. Not only that, but you get only part of the output when it's cloudy. The 1000 watts per square meter applies only when not cloudy.
I guess you could include let's say 4 solar cells in a foldable system. Then each cell could be small, and you could fold it to an opened form to cover the required area.
Also, very slow charging could perhaps work. The trouble is, most lithium ion batteries are designed to be charged at a rather fast rate. The battery may degrade if you are charging it at an unoptimally slow rate.
Also, unless you can find a charger that accepts DC voltage from the solar panels, you would need a small inverter to convert solar cell voltage to 120-230 volts. That would mean some extra weight and an additional component to carry around.
Buy few extra batteries instead, and if using DSLR, use the viewfinder and turn off the LCD to save the battery from exhausting.
Somebody proposed hand-crank, would that be a good idea? No! Most batteries are not designed to be quick charged, so you would need to crank continuously for at least an hour. My hands would become tired from that. A foot-crank system (similar to a bicycle) would be too heavy to carry around.
Fortunately, typically the supplied charger accepts 120-230 volts of voltage, works anywhere in the world, and usually you won't be out of reach of electricity for multiple days continuously.