If you are looking for a very compact enclosure that can handle a mini-ITX motherboard and up to two 2.5-inch hard drives while measuring in at just 192 x 210 x 62 mm, the Mini-Box M350 might be the performance case for your needs.
In looking for a low-cost yet suitable enclosure for the AAEON EMB-BT1 Atom Bay Trail motherboard, I ended up buying the M350 from Amazon.com for just $39 USD. The M350 has the claim by Mini-Box as being the "smallest, universal mini-ITX case" with a fanless, natural convection case design. The case is 2.5L, measuring in at 192 x 210 x 62 mm. The case is designed to be completely fanless for systems with having a CPU/SoC with less than a 10 Watt TDP while accommodating heatsink-fans for CPUs up to a 65 Watt TDP -- the M350 is tall enough to accommodate stock Intel HSFs.
The M350 also has two USB ports, an SMA antenna connection at the rear, and can support up to two 2.5-inch hard drives. While two 2.5-inch SSD/HDDs can be accommodated, the M350 only ships with one hard drive mounting adapter -- the second must be purchased separately. These adapters can also be used to fit dual fans should you need some extra ventilation within the chassis. Also sold separately is a VESA mounting adapter and wall mount brackets.