My recommendation would be to use riserless motherboards with their own cases with delta fans. If you have 3-slot GPUs, you should be able to remove the GPU fans and cool them with the case fans only (I did that with my Vega GPUs). Just make sure that the case is big enough to accommodate your GPUs and that it has delta fans at both sides.

A case with good clearance is the one sold by ITZR with its riserless motherboards (the same that Octominer rebranded). The cheapest would be to contact the manufacturer directly on Alibaba and get it imported by sea freight, although with the current world situation, transportation has become much more expensive.