I used a large supermicro server chassis, a dual Xeon motherboard with 7 8 lane PCI Express slots, all the ram it would take (bought second hand), splitters, four massive powersupplies. I extended the server chassis with aluminum angle riveted onto the base. It could be rack mounted but I'd hate to be the person lifting it in. The 3090s were a mix, 10 of the same type (small, and with blower style fans on them) and 4 much larger ones that were kind of hard to accommodate (much wider and longer). I've linked to the splitter board manufacturer in another comment in this thread. That's the 'hard to get' component but once you have those and good cables to go with them the remaining setup problems are mostly power and heat management.

Thanks that is very inspiring. I thought there are no blower type consumer GPUs, but apparently they exist!