Gentlemen, behold...
The 42U Rack. This model is not made by the same people who make our ships and modules. If it were I would be able to put a 42U case into it.
Or a 41U case, or even a 1U case, but once I did, I would not be able to put anything else into it.
Instead, this rack can hold a single 42U case, or a 41U case and a 1U case, or 10 4U cases and 2 1U cases, or any combination of cases upto, but not exceeding 42U.
This is Intelligent Design.
We have Design.
I'd be ever so happy to meet in the middle for semi-intelligent design - perhaps requiring a "spacer" of some sort, perhaps 1U in size, so I could still put 8 4U cases, along with 8 1U spacers, and still have 2U of space left - enough for one more 1U spacer and a 1U case.