That's more of a nitro box set up to carry a small 12v battery to be used as a glow plug igniter. You'd be better off with a deep cell marine type battery on a wagon or handcart. Or make an extention cord to run off your car battery. Do not charge both your batteries at once. It is very important that if you wire two batteries in series, to charge them at the same time, that all the cells have the same mah rating. You can count on a fire if you try to charge your two batteries as a five cell. As a 1/10th scale electric racer I can tell you that most guys get about 8-12 charges off their vehicle before they ruin their automobile battery with 2s 3500-5000mah batterys. I always start my car for and let it run for 15 or 20 minutes after every 6 battery charges. You can face the same problem with a deep cell. Lead acid batteries do not survive well if you drain them completely. Once again the little battery that fits in that box may not make it through even one charge, let alone a day of flying.
Your best all around bet is to buy a inexpensive 1000w generator in the $100 to $150 range (US).
Good luck!