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.
8 - 12 L
Silverstone Milo ML11 - 10.5ish L - might be worth an upgrade; rising it by 1 - 2 cm would allow for an SFX PSU, regular sized GPUs, and also an AIO (eg. the Silverstone Vida 240)
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)
14 L
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
15 L
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)
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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)
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16 - 19L
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
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Jonsbo V4 - 19L, could be a cut + crop candidate, would allow reducing its width down to 154 mm (regular width: 238 mm), ie. you cut away the part where the (ATX!) PSU would reside, and use an SFX/SFX-L, Flex or U1 PSU in the case front
NR200(P) - 19ish - official numbers range from 19 to 20L, depending on what parts are included into the calculation (eg. feet); allows for mATX mounts, either printed or just made from scratch using metal or wood, with SFX PSU located to the front only (similar to how its done in the SAMA IM01); in theory, parts and assets for the SAMA IM01 should fit in the NR200 and vice-versa
20ish L
SAMA IM01 / Inter-Tech IM-1 Pocket / FSP CST 350 Plus - 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! The FSP version is upgraded with a mesh front panel, which could help if you've planning to do a vertical build
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)
Jonsbo RM2 - 21.52L, but my guess is, with calculations going so wild across everything, its probably below 20L, eg. feet being added to the size, and the protruding PCI slot holder; if you remove these, you get 19.76; alas, it might be a very good cut & crop candidate.
25+ L
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
Blacklisted cases
There are a few cases that are not worth your effort, because they're nasty hotboxes, ie. thermals only gonna improve with either removing the side panels / whole parts of the case altogether, or hefty drilling / modification operations. Thats time, effort and also money you could invest in a much better case. So, without further addo, the list:
Jonsbo C6 - looks nice on the first and second glance, but is being reported as a nasty hot box, ie. removing the panels results in great temps, but with panels chokes the CPU and also the GPU
Raijintek Metis - could been a candidate for smaller sized mATX boards, but its being reported as a nasty hot box, too, which will only get better if you heartily add a few more openings
Raijintek Styx - is nice, but a cut & crop wouldnt help much, compared to eg. the Jonsbo RM2, which is of similar size, but better layout
numerous cheap 21 - 25ish L cases; those are all nice n dandy, but you cant really modify them enough to be worthwhile, for one, because of their build quality, which is mostly very thin wobbly steel, and also, because the result after cutting would need much more effort to really make it shine. Another topic is their internal structure, because they tend to have the PSU location on top, ie. extremely outdated design, which you'd either have to cut away or fill with a panel of sorts