Stabilizing the 12 volts from a car battery
I think the linear stabilizer will be acceptable solution, if your gadget may eat lower voltage than 12V (about 10% lower). If yes, then you may use LM-338 for example. This stability margin (10%) is needed because the every linear stabilizer has its own voltage drop near 1V or higher. And, of cause, follow the recommendations regarding capacitors - their capacity must be enough for expected load, and their voltage must be 50-60V at least to respect your remark about  real voltage pikes.
If your gadget is pretty simple and has some resistance to voltage overload, then you may use only LC-filter based on ferrite-ring and a combination of big capacitor and ceramic capacitor (purpose of second one is to eat the noise of generator's rectifier and ignition of brushes).
If the voltage precision is a key, then your choice is a double convert 12 to ~220 and then ~220 to 12, by using a car power adapter (inverter). These adapter should be able to give you much more than 2-4 Amps.