any 7.4V generator would fit.
Depends the size you need and how fast you need to charge.
An LP-E6 is about 2500 mA, so with 500mA you can charge in 5 hours.
on ebay you can find 6V solr panels for less than 5$, so 4 panels and a regulator hould keep you under 30. http://www.ebay.com/itm/2-5W-5V-500...ultDomain_0&hash=item5af1b58653#ht_8252wt_934
you mount 2 solar panels in serie and the 2 strings in paralle so you got 10V with 1Amp.
it is even better if you take 6V panel so you can use a cheap car 12V LP-E6 charger (but strangely 6V panels are a lot more expensive than 5V)
it is ok if you glue them on a nylon fabric, so you can fold easily.