I you had enough space inside your box 6 x 1.5v aa/aaa batteries will last a long long time, maybe years if you power a FF, Big Muff, dist+ or similar, depending on how often you use them.
I power my pedals this way if I can get a large box. The batteries in my Rangemaster and Fuzz Faces are 3 years old or so.
The ONLY problem you must be aware of is that sometimes after a long period of time batteries could begin to leak, so you must check your pedals from time to time.
The new two-dimensional carbon layer, graphene, developed by two russian scientists who were awarded the Nobel Prize recently, look promising. Can be used as photo-cell.