on the alloy thingie.. if not mistaken.. metals have a higher resonant frequency comapred to organic materials... thus, it may not work. Also since i have not seen a metal box b4 so dunno how it will really sound like.
On the DE plexi box... it would work as it does the same principle of mass loading the box and the deflex pad to disperse back waves... I did see one install on the net before.. the guy had a plexi box with the sub firing into a pile of sea sand... looks very nice and you ahev the imprints of the bass waves in the sand... very cool looking