I enjoy this type of build. Depending on your work space. They sell extruded aluminum meant for GPU Mining. It's got room for eATX mobos and space for 2 big PSUs. They are spaced far enough apart that if you get the 3000 rpm noctua industrial fans and line them up you can cool that many 3090s on a single rig. If you are willing to cough up enough for 6 PCIe 4.0 x16 risers. Remember they are finicky at best so make sure you keep your warranty papers.