There's also a device called the iSun ($80) from a Canadian company called ICP Technologies (icpglobal.com). It's a compact, easy-to-use solar unit that also is adaptable to charging a wide range of items. In its basic configuration, however, it's limited to 2 volts. You'd need to link three units together to get the 6 volts you need. ICP also makes a 12-volt unit, the BatterySaver Powerpak, that can be set to 6 volts and that has a cigarette lighter adapter. It's about the same size as the Diogenes, and retails for only $35, which sounds too good to be true, but there it is. I think the Diogenes has a little better solar cells, hence the price difference.
Keep in mind that none of these small units is terribly efficient. They'll work best in bright sunlight, although in Colorado you should be able to find plenty of that.