When you don’t mind packing some extra weight and bulk, sealed lead acid (SLA) battery packs are great. They last forever and recycle the flash very quickly (like fresh AAs). Just be careful when shooting at full power that you don’t melt your flash head! I also like that SLA batteries are supposed to be charging when not in use. This means they’re always ready to shoot when you are.
Making one is pretty simple if you can drill and solder. See for yourself!