For occassional use I'd suggest a small sealed lead-acid battery like that which you have found, as a common car battery charger should be able to charge it.

For regular use I'd investigate NiMH technology as used in RC vehicles. RC airplanes often use large cell count packs, so suppliers would be able to easily provide a 10 cell pack (Each cell is nominally 1.2V). But you would also need to buy a charger for it, so costs would mount. Depending on which size cell you'd be willing to lug 10 of, you should be able to get capacities from 2-7 Ah. With sub-c cells you should be able to get 3Ah, which should give you 3-4 hours playing time. (IIRC the empeg averages about 0.75A).