A metal enclosure will provide shielding for the PA3 controller, and protection from electric shock for the operator/user.
Any metal enclosure will do, but one with plenty room for both the controller and the 19 volt power supply is recommended. Also there will be a lot of cables in there, so too big is better than too small. A cabinet from a discarded desktop computer will do great, which you should find in any junk pile or second hand shop.
Here is one from Cooler Master, price: $49. There are plenty new and used enclosures on eBay.
The PA3 controller has a hole in each corner to attach metal spacers for mounting in the enclosure. This connects the PA3 grounding to the enclosure itself and is required.
You decide if you want to attach all wires directly into the PA3, or if you want to use sockets and plugs. The latter is more sturdy and looks better, but it is not required.