Well, I got a bit bored this past weekend and I was also tired of my car PC sticking out from under the seat (with the seat as far back as it could go on the rails). This is my case.. http://www.newegg.com/app/ViewProduc...108-176&depa=0
It is, or was should I say, 16.9" long. I bought a new hacksaw blade and chopped off about 6 inches from the bottom and sides. That whole bracket assembly up front is now gone. Then I cut off about 2 or 3 inches from the top, curled a bit of the metal over to make a nice smooth lip (jaggedy edges suck) and bent that down over the case to form a face. Now it's only about 10" deep and can fit under the seat perfectly concealed. Nobody will be kicking it anymore! Here is a pic of the finished product (note - the screws, which are now painted black, are holding the HDD in place, and I have since cut fuel line and wrapped it around the other three jagged edges).
It's pretty neat comparing the size of that to the size of my regular PC. It's no miniITX or anything, but it's pretty durn small. One thing I learned from this little project though was not having the right tools sucks, and when I say right tools, I mean power tools. You'd think aluminum sheetmetal would be easy enough to cut through, geez.