Tiny Power Supply

I’m gearing up to finish a project I started a while back — putting an iMac’s innards inside an old SCSI drive case. Right now I’ve got it working with the original (huge) PSU and the logic board stuffed inside the thing using cardboard to prop up the logic board, and the HD velcro-ed to the back of the chassis (on the outside). Very inelegant.

I’d like to get rid of the original PSU and replace it with something smaller so that I can actually get everything inside the case, including the HD & CD (there is no CD right now,) but looking for power supplies has been frustrating, mainly because I don’t know what I’m looking for, where to look, or what might or might not work with the iMac. A mini ATX PSU might work, but it’s still a bit on the large side for my case (which is 9"x9"x5"). Even the PSU’s I’ve found for 1U cabinets are a bit largish; they have a low profile, but they are still too big to fit in my case. I've also found some small PS boards without the metal enclosures at http://www.microatx.com, but the prices there are a bit more than I'd like to pay. Ideally, this will cost me on the order of $20 tops. Has anyone bumped into anything that’s on the order of 2″ high x 3 or 4″ square (or thereabouts) that is fairly cheap?

Another thing I’ve considered is using the PSU from an older Macintosh; I parted out several LC’s and 61xx machines a while back and still have some of the PSU’s from them. Would there be any problem using one of them on this project? The only thing that I could think of that would be problematic is the rocker switch and lack of soft power on those particular PSU’s. Although in the role that I plan for this machine — an always-on mail server — that isn't a huge problem. Any other old Mac power supplies that might work, and be able to be hacked to do soft power?

Any idears?