12v to 12v PC power supply

[crashwg]

I'm seriously considering putting a computer in my car and I came across this http://mpegbox.net/MPBS1_Spec.pdf which looks pretty cool, but expensive.  I'm wondering how hard it would be to make something like this.  I don't have have a whole lot of electical experience so I probably couldn't design the circuit, but I'm pretty sure I could solder it all together.

Does anyone here think they could design something like this?  Or maybe know of something similar but cheaper?  I know there's the option of an inverter, but it doesn't realy make sense to go from 12v to 120 and then back to 12 if you don't have to...