I have got an empty case from an Indigo² standing on my desk, next to the PC that I use.
I have been planning to build a powerful graphics PC inside it, but the problem is that it is a "pizza-box" case that is only about four inches (10 cm) thick inside, so using modern coolers is difficult. There aren't many good fans that would fit, and I do absolutely not want to cut new openings. I have been planning to modify a watercooling radiator to fit.
An Indigo or Octane case would be easier to convert as they are more cuboid. I have seen PC-conversions of those, of O2s and of Indys on the web, but the Indys have been mostly low-power mini-ITX systems.
But please do only do that if you get hold of an incomplete system that does not work, and do sell off the extra bits that you don't need. There are vintage computer collectors who really like these machines, and throwing away components would to them be like throwing away a nice vintage keyboard is to us.
My machine would be built as a tribute to že olde SGI - that is why I want to put a nice graphics card inside it.
Another idea I have played with is to rebuild only the exterior of a boxy PC case such as the Corsair Air 540 or 240 into looking like a miniature version of a first-gen Onyx.
There are a few other people on this board and on Deskthority that have SGI's. I have seen that 7bit has an Indy on his desk, but I don't know if he uses it, has it converted or uses it only as a monitor stand.
A few guys in UK and Ireland got together to buy a bunch of O2s a year or two ago.