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.
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.