Scott - I use ordinary glass and silicon, then put it in a box, though I suppose that plexi is much less fragile! The fixing bath doesn't need a box - and hypo (what I use) is cheap, so a tray will do.

You can use a tray for the silver bath, especially for odd/large sizes - see here.

Joe (aka "Smieglitz") made a neat vertical silver bath out of red perspex, which doesn't need a box either.