Castle Creations makes a BEC that is around 21 bucks. Will take in up to 25 volts, and output is variable from 4.8 to 9 V, with the default set at 5.1 ~10c. I think that is the way to go, then get a 2S or 3S lipo and power your plane for a month without recharging.
However, remember a special charger is needed....
Also, Lipos don't lose charge sitting around, so you can charge them when you return home, then they can pretty much set for a month or more before you use them again, and still be in good shape for flying.