I've seen requests like this come in from campers and sailors all wishing to power the equipment off-grid using usually very limited supplies.
Using an Inverter & the supplied AC/DC adaptor will work, but is very inefficient. Not to mention that cheaper inverters can actually damage sensitive electronics if they produce a very poor sine or simply pump out a 2-step square wave...
If you get an inverter, expect higher than necessary current draw to power your device, and please don't cheap out unless you only plan to run a light-bulb or hot-plate.
Harald's suggestion is most likely going to be the best bet here.
We don't know if 14.4V will harm anything... you can build a simple regulation circuit with nothing more than a resistor and zener diode.
If you can share the current draw in mA, that would go a long way in suggesting a better alternative.
(If current draw is too high for the simple method, you can find/buy a 'buck' type switch-mode voltage regulator that will give you a clean 12.3V output and many are adjustable.)