Comment by danjoc

Here's a few choices,

https://www.amazon.com/SOKOO-Portable-Efficiency-Reinforced-...

https://www.amazon.com/Foldable-Waterproof-Efficiency-Cellph...

https://www.amazon.com/Balight-Outdoor-Foldable-Powered-Char...

28W basically gives you an fourth panel, but your backpack is probably 3 panels tall.