You can now easily adapt any new post-2012 iMac for VESA mounting

[iMore.com]

NewerTech now has a new adapter that makes it much easier ? nay, possible ? to mount any 2012-or-newer iMac, even the new Retina 5K iMac with the universal VESA system. The NuMount VESA is a machined aluminum bracket that clamps through the power-cable passthrough in the iMac's stand, adding both a quartet of screw holes for a 100mm x 100mm VESA mount and a shelf above that for storing items you want nearby but neither want to see or need to reach (say, an external hard drive for backups).

Full story from the iMore Blog...