Icy Dock has built a new hot-swap bay. This one, which is to be known as the TurboSwap MB171SP-B, is a unit that fits in a 5.25" bay and can house a 3.5" drive.
The unit is fully made of metal and has a number of extra features above ordinary 5.25" hot-swap bays. For starters it has a cooling fan; for the main course it has a built-in fan controller for that fan; and for dessert it has a locking mechanism.
The 80 mm fan can also be swapped for a 120 mm fan, though that will extend into the space below the cage. Power for the fan is taken from the SATA power connector that is also used to power the 3.5" drive.
The locking mechanism is done with a key and a barrel lock, which you should hide somewhere you won't lose it.