The only limitation is that these PSUs top out at 300W so I wouldn't expect anyone to throw in a 9900K with a 2080 Ti along with them. It is possible to make custom cable sets that would plug into the 24-pin connector, to make the lengths fit better inside of the case. I like these PSUs because their 24-pin modular connector is at a right angle, and it sits within the 150mm length (The M1U is too long being 190mm). The types of FlexATX PSUs that would fit are 150mm non modular PSUs, with one main exception. I know there are some very slim angled C13 power adapters that come from Taobao and I might try those out. Not to mention the angled power plug coming out from the bottom, would need 20cm of room there. It is 170mm long because of the modular connectors, and the attached cables need at least 12mm more room. The Geeek PSU that you mentioned would unfortunately be too long to fit inside this case as it is. Figure there could be a need for cases that would more affordable than the Dan or Kolink cases, if they are fine with choosing one made of plastic. I was meaning to make 3D printed cases with a sandwich layout for quite some time. I had split the case in order to make it more manageable for printing time. Thanks for your feedback! Right now looks like design is still under review by the shop but I'm confident that it's easily printable.
