You mean 12v 5A? That would not be enough power. Usually those boot adapters are 10A though, and that would probably work. It would be best to run your own wire to the battery though, (with more like a 15A fuse) because you never know what is running to that lighter adapter and if it would work. The equation is volts x amps = watts. Search the forums for examples of what others have done.
The basic choices are a dc-dc supply like the opus, or a dc-ac inverter of your choice. The inverter option is inefficient because it converts dc to ac, and then the computers psu turns it back to dc for your parts. Because you are using a mac, however, you may find it easier to go with an inverter. I would avoid that lighter plug though, if it is only 5A.