Take a closer look at some of the higher quality cases.

All the cases I have purchased within the past few years have plastic, as
you say, over the drive bays, but there is metal behind that plastic. If
you want to use the bay you need to pop out the metal. Except for floppies,
the drives generally have metal behind their plastic faces, too.

More important is the scheme for attaching the sides. Those little drive
holes will let some RF out, but long, narrow gaps in the side let out a lot
more. Most recent cases seem to have fairly elaborate schemes to be sure
that the sides don't leak.

I can't believe many of the mass marketed cases actually make it to
manufacturers who need to be certified, but the lengths they go to ensuring
RF shielding lately would indicate to me they are getting pressure from
somewhere.

As best I can tell, the case isn't nearly the problem the wires are, but
even there, it seems like the newer stuff is pretty well behaved. My
radios are pracically surrounded by computers and no noise anymore. Used to
be a real problem but as the computer equipment has gotten newer, the
interference problems have faded. I can still hear my Ethernet on some
bands, but I have a pretty ancient hub. Once that gets replaced I suspect
the ethernet problem will disappear, too.

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