The Numato Opsis is compatible with the Mini-ITX standard and should be mountable in any standard Mini-ITX case.
As Mini-ITX is a subset of the ATX Standard the Opsis board can also be mounted in any ATX case too.
Inset Technologies Thin Mini-ITX Computer Case
The Inset Technologies "Thin Mini-ITX Computer Case" is a great case for mounting the Numato Opsis in and is used by a number of the TimVideos.us team.
The case is a 1RU high, half width which makes it perfect for mounting in A/V racks. When combined with the 1U Rack-Mounted Faceplate it can also be mounted in full size racks.
A 3d printable version of the I/O shield can be found here.