Doesn't look mentioned, but a Custom_MOD FLEX mini model could work. The FX mid stl 4.7L fits all your requirements (<5L, flex atx, 48mm cooler), the only issue is that I don't see it for sale on their form, so you may have to reach out! Else the FLEX mini 4.5L could be satisfactory as well (39mm cooler though, so maybe not).
