This isn't that hard, finally found a good schematic. The biggest problem with having it on the Ignition (switched) line is that it will cause hard shut downs, and ideally you want to let the RPi shut down naturally so your SD card doesn't get corrupted.
Anyway, go to this page: http://www.obdsol.com/stn1110/
Down at the bottom, check the STN1110 data sheet. On page 12 figure 6 shows a protection circuit - this is important in a Car PC situation. Secondly you see the "5V" and "3V" boxes.
For the 5V, check this out: http://ww1.microchip.com/downloads/en/D ... 22284A.pdf
It's a 5V, 3A output buck regulator with over 90% efficiency. The PDF includes a recommended schematic/layout.
My plan is to use these, at any rate. Connect the power to the 12V batt line and have the protection circuit in front of everything. 12V from there to a touch screen, 12V into the 5V buck regulator, and probably have another 3.3V regulator as well. Have the switched line acting as a shut down trigger by having it connected to a timer that will then cut power - when the EN input on the buck regulator goes low, it turns it off - cutting power to everything after it. Just need to get a timer that can handle 12V. Might need a second 5V buck regultar with circuit protection, unfortunately. Haven't figured this part out. I guess an alternative would be to run a physical switch to the EN line, so you have to manually power down.