Well the metal plates on the front and back are exactly the same size as those on standard size G2 steelbooks. It's just the plastic housing beneath that is jumbo sized.
So you'd remove the plates and fix them to the plastic housing of a standard size steel (that you've already removed the original steel plates from), then add custom interior art and spine to match.
I've not removed them myself, so I can't tell you first hand how to do it. There are plenty of guides about though. I've just bought versions from people that have slimmed them down themselves.