No one makes such a thing. The toughest aspect would be the cost of paying a fabricator, as it would take a lot of technical skill to make it yourself.

There are also two other problems:

(1) Unlike the original G5 version of these iMacs, the Intel-based ones have the side and rear part of the case as one piece - so you'd need to make the front bezel and the side/rear (whereas with a G5 iMac you could just make the front/rear piece clear, which would be easier).

(2) But the biggest drawback by far is that the Intel iMacs (again, unlike the G5 iMacs) are butt-ugly inside - the internal components aren't arranged very nicely, and there are sheets of metal EMI shielding all over the place.