Ah, a powerful 1-DIN 12V powered PC!
"powerful" is the key word...there are 1 DIN (or smaller) PCs, but they can barely run Windows XP (a sad litmus test for "powerful").
Consider *NOT* trying to put the PSU inside the computer.
I have a hack that I am propose for the Travla C-134 on this page at my site:
It's a tight squeeze, and cooling might be an issue, but it might be worth checking out.
I also recommend some power options that might help you out:
That PSU they use in the Bantam IS one of, if not THE, smallest PSUs on the market:
Even then, I think it is too large to make a 1-DIN ITX case possible.
So, definitely consider mounting the PSU externally.
I am looking in to a 2-DIN PC myself: