For such a purpose, I go with a lantern. Right now, I've got a Mallory (out-of-production) with a swivel head and an amber blinker on the back, uses the 6V rectangular screw-top batteries (I've made up my own battery using a 6V 10AH SLA in a housing that clips onto a bracket/charger so the lamp trickle-charges whenever the engine's running).
I've also got a few cylindrical (about 4" diameter by a foot long) "4-way" lights (hotwire beam out the end, flourescent out the side, and either single or alternating blinkers opposite the flourescent) that take 6D batteries (2 stacks of 3). These are available in both "silent" and "noisy" (when in single-blinker mode, they also run a beeper) versions (all of mine are the "silent" variety), and have a built-in charger for NiCd batteries (although on 7.2 volts, the flourescent won't light properly). On the other hand, with a couple of 12AA to 3D adapters wired for series-parallel (3 stacks of 4 cells) or (got a good deal on them at the local surplus emporium) stacks of 16 4/5AA NiCd in a 4x4 series-parallel, they should provide a good, constant-charge lighting setup.