I wouldn't recommend a DC-DC converter at all.  Put your money into a good RC battery pack and charger.

A 3-cell Lithium Polymer battery would work perfectly, as they usually store between 11.1 and 12.6 volts (Google around, hobby stores like Horizon Hobby carry these for RC planes and helicopters).  No DC-DC converter needed.  A balance charger might be a good idea.  Li-Polys have excellent energy density to weight ratios.  The only thing you have to watch out for is over-discharging them (well, that and explosions ).  If you're worried about that, try to find some A123 batteries on Ebay (Lithium Iron nanophosphate), that can be discharged down to zero volts without harm... but they're still expensive and hard to find.  NiMH batteries work ok, but they're heavy in my opinion.

As to whether any of these battery types will fit in the case really depends on how long you want to run it (larger capacities = larger batteries).  A 1320 mAh 3-cell pack would probably fit ok and last a few hours (I don't know what the power drain of a Chumby is though).