Don't forget the Eocene fossil Crassostrea gigantia for thickness.  About 
ten years ago when we were at Shell Bluff (GA) my younger son found just
the hinge of one.  It is about 200 mm long and 75 mm thick not including
the shell it is cemented to.
As to fakes, they are made to look good in reflected visible light.  This 
plan imposes limitations that can be exploited by dealers and collectors.   
1) Fakers must concentrate on appearance under reflected visible light and
have less time to worry about other lighting and 2) techniques that are
successful in reflected visible light usually conflicted with success under
other lighting.  Hence, the suggestions people have been making.
Tel-Atomic in England makes a very nice, safe desktop X-ray machine
available for 2 or 3 thousand dollars from them and a number of scientific
suppliers here in the US.  I can provide names and addresses to anyone
interested.  The machine is about the size of an automatic bread maker and
is as portable.  I have not used it with shells and it does not have room 
for large ones but it would hold an cowrie or cone I have seen.  It does
not produce high intensity beams but more exposure time will compensate.
I think the cheapest and best idea is a very strong visible source.  Again,
such stuff is available from scientific supply houses but also from other 
sources as well at prices significantly below the X-ray machine.  But - for
very thick shells, probably nothing will work well.  Very bright UV source,
neglecting the Sun, are not generally available - some very expensive UV
lasers perhaps.  The problem with UV is recording an image will require
more special supplies and film.  At least X-ray film is fairly easy to get.