This question is really aimed at computer cases, but I suppose it also applies to other PCB cases. Most of the Computer cases I come across are aluminium, or else some other metal; As I understand it this is because of the following properties:
1) thermal, a metal case will nicely dissipate heat applied to any point, and conduct it to the cooler areas (outside). Also, doesn't burn (though many cases have plastic on them too)
2) RF, a metal case will shield from outside interference, and also keep inside EMF from interfering with other things.
3) strength, to keep heavy equipment in-place and protect them.
4) earthed, add a level of electrical protection by having an earthed metal case around components.
My question it this: why can't other materials be used if these are the only concerns? Other materials may be cheaper, easier to get or work with, lighter and importantly cheaper.
For example, Wood has good properties for #3, it can be strong, and servers don't need to be portable much, so it only needs to be as strong as not falling apart when just standing there. #4 isn't really needed, #1 is important, but most computers use fans and heatsinks (rarely the case itself is used as a heatsink) already, so with well planned airflow the cases needn't also be thermally conductive (maybe thermal sensors may be more important though); which leaves #2.
Now, would it be possible to use some kind of metallic tape on the insides of the case? I've seen thick RF tape, but that stuff's expensive. As far as I know, aluminium (baking) foil is actually mainly plastic with a thin layer of aluminium. I've seen cheap aluminium tape (for the roof) but I assume that's much like the baking foil; Just how thick does the layer need to be, and can it be done cheaply? Could I even use something like chicken wire?