Solio makes a few good portable ones.
I use this
Solio H1000 Universal Hybrid Charger (Orange) and strap it to the outside of the backpack. The only problem is if even one bar gets shade on it it, the thing will not take a charge. Bonus is that you can charge it USB and have a full charge before you go.
keep in mind i got mine a few years back so im sure there is better ones now a days.
Might also want to look at REI.com and see if the fold out kind is more suitable for your needs... like if you car camp.