We already provided additional holes on top and bottom of the case for fixing custom PSUs. I'll reply with drawings little bit later.These are exactly my thoughts too.
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If you look at the pictures posted in this thread, I think you would only need to add two holes, tops. It takes 3 screws to mount a typical Flex PSU. I don't know the current distance between the holes, but it may already be aligned with 2 of them.