I used to run a PFSense ... like Supermicro has a 1U chassis with board for about $270 which includes almost everything you need. I'd even make my own though, I've seen some tiny mITX cases.

Or, just buy the parts integrated by SuperMicro ... motherboard on my list are different. My case has front-facing I/O, and my motherboard is a X7SPE-HF, which is slightly longer than the Mini-ITX ...