A short list of PC cases smaller or around 20L of internal (!) volume which are compatible with the mATX (Micro-ATX) mainboard form factor. Availability focuses on Europe.

• Silverstone Milo ML11 - 10.5ish L
• Inter-Tech S-301 - 8.55L
• Inter-Tech S-331 - 9.11L; nearly identical to the S-301, but supposedly 2 cm longer (probably just a different front panel)

• SSUPD Meshlicious - allows for full ATX boards, with just a few small modifications (also see my build thread in the SmallFormFactor forum, where I go into detail about the topic)
• SSUPD Meshroom S - basically the upgraded version of the Meshlicious, allows for full ATX without any modifications; has almost all of the mods and workarounds built-in from the start

Originally I tried to search for a PC case thats a bit bigger than the very attractive, but slightly too thin Silverstone Milo ML11, but eventually stumbled upon a 15.5L noname case on Amazon, which sparked the hunt

• First found via Amazon, a no-name case with a TG side panel, 15.5L, available from Bewinner, among others
• Further digging brought up a case by TDGamer (US), without TG panel (ships from US):
• Same is available from Mars Gaming, but only WITH TG panel: MC-Core / MC-CoreW (White version)
• A very similar version of this is the Mars Gaming MC-1000, but with just 14.5L
• Tacens Orumx - Similar in size, if not even the same case; without TG panel (available in Black or White, White is full case, not just the panels)
• In theory, V2 of the Dan Cases C4-SFX (whenever its gonna be released)

• Itek SMALLCOM-P / SMALLCOM-B etc. - 18L tower, probably from Italy; its available in three variants, which are sometimes easily available, and sometimes get listet as “in 3 - 7 months”; same company also produces a clone of the SAMA IM01, and is one of the few manufacturers of cube cases
• Jonsbo V9 - 17L, with TG sidepanel, full aluminium case, only weights in at 2.7kg
• Mars Gaming MC-S1 - 18.64L (also available in pink)
• Mars Gaming MC-S2 - 19.43L (comes with semi-mesh front)

• SAMA IM01 / Inter-Tech Pocket IM-I - 20ish L; its 20ish L, not 21 or 22, because they add things like protruding elements to the 'calculation'; according to this 'calculation' (dyscalculia anyone?), the case is 2 cm longer than IN REALITY!
• Mars Gaming MC300 / MC300W - 19.95L / 20.11L (there are different versions of the same case with the same setup but slightly varying measurements, so its probably the front that extends a bit further)

Potential cases for “cut + paste” or “cut + crop” actions with a variety of powertools and manual labour, but also decent (interim) cases for different projects:

• Mars Gaming MC400 / MC400W / MC-PRO - 24L cube case that fits almost perfectly inside a kallax shelf; in general, Mars Gaming got a lot of those smaller sized cases in the range of 14 - 25L, you just have to click yourself through their Amazon shop or pages
• Chieftec Uni-UK02B / W - 30ish L ATX cube case, ie. fits a full ATX board; only weights about 3 kg; could be a good case for size reduction, eg. using a dremel; cut away the right chamber, get an 18L case as a result (could be reduced even further)
• Mars Gaming MC-View - 32,1L; a slight upgrade of the MC400, as a fish tank case, clone of the classic Lian Li O11 Mini (available in black with pink elements, too)
• Asus Prime AP201 - 33L; its such a nice case to work with, whether as an intermediate option or permanent. I own this case, yes, but I would consider a bit of cut + crop with the white version, although it would probably only result in an 19ish L case