Supermicro H13SSL just slightly exceeds ATX size, by 0.5" = 13 mm more depth. Should fit into many ATX cases.
(ASRock Rack GENOAD8UD exceeds ATX depth by 0.9" = 23 mm. This board is harder to find in retail, possibly a bit more pricey, and limited to 8 DIMM slots.)
Cooling solutions: Depends on what TDP you have in mind.
The only existing decent DIY cooling solutions for the larger core count Genoas for now are custom liquid cooling loops (if you accept them as 'decent', considering their caveats). However, I wonder if the VRM banks are sufficiently cooled with low air flow like in typical DIY scenarios. DIYers could certainly build their own liquid coolers for the VRMs though, by modifying off-the-shelf liquid VRM coolers, which aren't easy to source these days though.
Noctua "next-gen AMD Threadripper coolers" will be out Real Soon Now, but it remains to be seen if they will be for SP6 or for SP5 or either one, and what they really will accomplish if you don't strap high speed fans onto them.
Edit:
The thing is, a heat flux which previously was happening in dual-socket systems now needs to be dissipated from a single socket. Of course server ODMs have their solutions for that, but these are neither quite attractive to, nor easily obtained by DIYers.
I wouldn't count on Noctua to come to the DIYer's rescue with a solution which has got the amount of radiating fins and heatpipes of two NH-D15s soldered onto a single coldplate.