A 9V battery connected with polarity proper to a 12V cigarette lighter plug should do the trick. Unless you modified it, the Riv's cigarette lighter is hot all the time (NOTE: the acc jack on the side of console is not). See THIS LINK
However, a 12V battery pack such as LINK
connected in place of that 9V battery will almost certainly work - will run longer (more watt-hours due to bigger battery). Such a battery pack will run a portable 5W handheld CB radio for instance and you can buy the batteries for $2 (4 pack of heavyduty batts usually $1) at a "dollar store."