First and foremost problem: power.. heres my proposal, a black & decker electromate 400. In case you haven't seen one (available at walmart), you charge it up and have 110 volts to go. Its designed to power small tv's and appliances. Put that baby in the trunk with the cpu, plug the electromate into a 12 volt converter and it recharges while plugged into the 12 volt supply. The best part is that if you shut off the ignition, like stopping to put fuel in the car, the cpu stays powered up! No shutting Hal down unless your parking for a good while.
The erector set is a good idea, or even stock aluminum from the hardware store machines pretty easily.
Blue Tooth? Hmm... good thinking. I suppose you would attach the blue tooth receiver to the usb slot and sync it to the headset? Will it allow audio transfer? That would be awesome! Blue Tooth works upto 20 feet isnt it? I can see me coming out of the grocery store, arms full and giving the command to open the trunk! HaHa..
Heres my ride, 2008 Toyota Yaris Sport (don't laugh.. its paid for!)

As you can see, not much room for dash monitors.. although modification to the radio display may be possible

Perfect spot for the power button and reset or harddrive led?