Li-Ion NP1 style batteries, battery cup with hirose connector and dual charger would be a good external power solution for SD 7xx.
They are a nice size for most audio bags and the form factor has proven itself through the years and various battery chemistries. They are readily available so there's little worry about them being discontinued when it's time for replacing them but they can be re-cell'd too if you want to go that route. On top of that there are lots of accessories available for them including distribution boxes as your equipment grows
They're not cheap though but are a professional solution worth investing in just like your recorder.
IDX, Varizoom, Swit, Frezzi, etc.