Unfortunately, not much can be done yet. Apple did do something special with the form factor of the Mp that PCs can't do because of established standards. This came at a cost, of course, with there only being a single proc in there and being 300% reliant on thunderbolt, but nothing PC can even try to compete and make a cylindrical option. There just isn't a standard form factor that allows someone to arrange the bits in the same manner.
You could go for a micro ATX motherboard setup. It will get you the closest, but also at a cost. Typically, you can only use one good video card in there, which is fine, but it just won't ever be your octane farm or anything. They also typically have fewer PCI slots than the typical workstation-class motherboard. It may be perfectly fine - especially if its a mobile rig. You can still house the same procs and ram as the large boards.
Here is a link to micro-atx cases. Since form factor seems to be the largest concern, I'd say find a case that would work best for you and build around that.