I am building one into a MDD case and am reusing the 4 drive carriers and the PSU. I am using two Lacie SATA hubs to go from thunderbolt to 4xsata. The MDD case is currently running the drives but I have yet to fit the mini itself. Eventually I will probably rip the power supply out of the mini and feed that 12V from the MDD powersupply as well.
Strictly speaking of course, I am leaving the mini in its case (with the exception of the PSU). It has two internal drives which will remain internal, I will just fix the mini case inside the MDD case.