The tube used in the Quickie is often fairly microphonic - a consequence of the tiny wire filament that gives low battery drain. In addition to cutting the cost significantly, the acrylic is better damped mechanically. If you use metal, give some thought to vibration isolating tube sockets. You may have to invent such a thing, old stock are few and far between, and probably stiff with age to boot!
Make sure the RCA jacks are isolated from the chassis if it is metal; there should be only one point at which the chassis is connected to the circuit ground - T3 which is adjacent to the tube sockets will do so when it is mounted, and T8 is not used so it won't be a problem.
The presence of a conductive plate will serve as a ground plane, reducing sensitivity to electromagnetic interference.
Don't forget to read the sticky about reversing the D-cell connections!
