How to fit a M1-ATX

[hcccs]

I've just bought my first used mini PC, which consists of a Travla 134 box, main board, DVD player, hard disc and that fills up the box. It has a small socket for 12V. The question is, can I connect the 12V socket to a car battery or not? Voltage from car batteries tend to vary considerably. I also have an ATX power supply (M1-ATX) and perhaps I should use that. The problem is that I can't see how I could squeeze it in. Advice please.