It must be metal. A basic of building good radio equipment. It keeps unwanted RF out and in.?
Shielding is also a basic. Take a look inside any commercial transceiver and you will see it. Lay out is important too, keep inputs away from outputs.
I have built uBitx into the case of an old Kenwwod TS120S and and it has worked out quite well.
Roger ZL2RX