the best way to have a laptop behave like a carpc is to solder wires directly to the power switch. it is very intimidating but it is also pretty easy, & once you do that you'll never want to think about any other option. you can get a cheaper startup shutdown controller or even a cheaper small atx supply that will also give you 12v & 5v & also do the startup automaticly. even if you just wanted a push button on the dash you will need to go to this switch...

you may also have to disable a lid switch, some laptops won't even start up when the lid is closed. if you want to put up any pictures I'd try to help guide you. if your really dead set against opening the laptop then I guess search & see what your other options are, but to me most times there are no other good options...