risky? its just two wires, follow their instructions, get the right power connector (looks to be a '6 pin DIN' connector, like this: http://www.amazon.com/6-Pin-DIN-Plug-240-b0/dp/B000LDWV80
), get a few feet of 2-conductor 16 gauge wire (lamp 'zip cord' would work fine), put battery clips or a powerpole connector on one end, and wire the other end to the DIN connector according to the operating manual appendix, plus goes to pins 1,5,6, and minus goes to pins 2,3,4.
If you're not comfortable soldering electrical connectors, find a radio amateur or something, I'm sure there's tons of people around this community who could make you a solid reliable cable.
the manual says 12-14VDC at 6 amps with the cooler on max, so you should use a fairly large battery, like a full sized marine-RV battery, this will give you enough power for an all-nighter (100-120 amp*hours rated battery, so 12 hours at 6amps is 72 AH, that won't over-discharge the battery).