Car Adapter?

[Zion Lion]

The only thing i miss on my gp2x is a car adapter to power it even during long car trips. It's shouldn't be very difficult to build, since the current that the cigar lighter provides is already DC and 12V.
The voltage should be only downstepped in someway to 3V (or more, since i read around that the gp2x works also with higher voltages)...
Has anyone ever tried to build something like this?

 

[aapje89]

why would you, you can just buy one in the shop...

I have one, you can change the voltage and the polarity on it and it has various plugs .