Card15 Card16 Card17 Card18 Card19 Card20 Card21
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Card 8 Card 9 Card 10 Card 11 Card 12 Card 13 Card14
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Card 1 Card 2 Card  3 Card   4 Card   5 Card 6 Card 7
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PSU1/SSD              MAINBOARD              PSU2
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So look at this and make it modular!
If someone has 7PCIE-ports they use just the two lower layers.
If someone needs 7 more GPUs they can put a second frame on the first and fix them together (layer 3).
If someone needs another 7 more GPUs they put another frame onto the second frame and fix them together (layer4).

Maybe this idea will help you