IMPORTANT QBX's ultra-compact format requires a Power Supply Unit that conforms to certain format standards. Please pay attention to a Power Supply Unit's AC Socket and Plug's orientation and confirm it corresponds to one of the formats defined in the pictures below.

They cannot be added because of the plug orientation limitation it has been asked quite a few times.

https://pcpartpicker.com/product/3V648d/cougar-case-qbx

I thought about this case for a while, since it's a pretty small budget case that can accommodate my 320mm long MSI Duke 1080ti. However, the PSU situation is pretty annoying. The PSU orientation issue that Gilroar mentioned was one of issues, though it's not always a major problem, unless you really wanted to use the slim optical drive bay.

However, in addition, if you install a graphics card of typical-length for a dual or triple fan card, PSUs must be both under 140mm in depth (as already specified) AND must be nonmodular, or else the connectors can be blocked by the body of the card. With a card that's on the shorter end of the spectrum, perhaps one could get away with a semi/fully modular unit, depending on the exact length of the card, the placement of modular connections on the face of the PSU, and which connectors are needed. It's quite a headache to sort that information out, and not something you could ask PCPP to do at this time.

You could use a bracket for SFX PSUs, though. Perhaps some modular SFX-L PSUs could get by with a longer graphics card, but there's no guarantee. For me, one of the original selling points of the case would've been the ability to install a generally less-expensive ATX PSU.

If you end up considering this case (or some other ones with poor PSU clearance) and still want an ATX PSU capable of supporting a high-end build, and want modular cabling, I'd keep an eye out Silverstone's 125mm deep 750W Strider unit, or EVGA's Super G7 line. (Both of which are supposed to be in the works.)