K39 is sub 5L Mini-ITX case. The 4L K39 v2 was discontinued and replaced by 5.2L K39 v3. Version 3 is only available in white. K39 v3 is still one of the best SFF cases in the 5L category due to its entry price and hardware compatibility.
IQUNIX ZX1
The ZX-1 is aluminium unibody frame Mini-ITX case starting 9.2L internal volume for the air cooling version and 10.6L for the liquid cooling version. Case pricing starts at US$219 and ships to most countries
Fractal Terra
Fractal Terra is a small aluminium sandwich layout Mini-ITX case at 11.4L in volume. It features swing arm, quick release side panels and adjustable internal spine for better hardware compatibility. It’s priced at US$179 and availability in most regions.
FormD T1
FormD T1 is widely considered one of the best small for factor PC case for both versatility and compatibility, while still coming in at under just 10L in volume. The case provides satisfying build experience and multiple configurations, such as air cooling, AIO and custom liquid cooling.
Densium 4+ v2
Densium 4+ v2 is 5.5L Mini-ITX case, constructed of aluminium, stainless steel, with woodgrain front panel design. The Densium 4 and 4+ v2 cases are only available from Overtek.