Last minute LED xmas lights running off a battery for a costume

First time posting here because I'm a novice and you folks seem to know your stuff! I'm about 48 hours from a big event (SantaCon NYC) in which I plan to have Christmas lights flashing on a costume. Problem is, I can't figure out how to get them to work. I have a Black & Decker "Pocket Power" brick which has an AC outlet on it, so that would have been perfect, except it doesn't hold a charge.

The lights I have are on a 10 yard string of white LEDs. There's a hundred lights in series in total. They are controlled by a small circuit board in a plastic casing. The box says they use 55W and have a frequency of 60Hz. The end has a standard 110V two prong plug. So I need something that will power them for roughly 4-6 hours as they flash and twinkle.

From a day's worth of research on the internet, I've found that the easiest way to do this would be connecting a battery pack to a power inverter which would change the DC juice from the battery to AC then allow me to plug the lights directly into an AC type outlet. I need to know what kind of battery I should get and how I can connect that battery to the inverter since most of them have a car cigarette lighter type of connection. Ideally this should all fit into a large pocket or fanny pack.

Since it's getting down to the wire I'm starting to freak out a bit, and I'm a novice with electricity at best. Any advice would be greatly appreciated. Thanks in advance.