why would you spend the time on a circuit if u can get an inverter much faster, as well as cheaper. ypu dont need to worry about the sine wave issue, as its going into another transformer to the laptop, which is further smoothed out by the capacitors in the AC-adaper. if you are worried about back surges and the like, most if not all inverters are prepared to handle that, as my cheap one even is, but if you witsh, you could add a surge supressor/ UPS and further lengthen the ability to supress teh minute surges that MAY occur. i have been using my inverter for 2 years with several cars, in a 97' mercury villager, an 87' honda civic, and a 74' ford pickup. iv had No problems whatsoever in any of these vehicles, and neither should you... and if you DO then its probably because you are exceding the wattage ratings, if ur using like a oven or somethijng, not becasue of surges of not the correct wave forms. AC out of the wall isnt always perfect either, the wave forms can be "deformed" as well. i know my friend lives in an old are of town and needs this box thing hooked up to his monitor to reconvert the signal from whatever it is to a since/square wave otherwise he gets lots of "fuzz".
USE AN INVERTER!