My favorite humidity chamber for slides is a stack of lids to 24-well
polystyrene culture plates.  They hold 4 slides, or 3 with a rolled up
Kimwipe saturated with water at one end.  Many have raised rings that can
also hold little puddles in place and keep the slides separated from the
bottom, which makes pickup easy.  They're made to interlock so you can make
a stable pile as high as you need with a small footprint, they're clear so
you can see that specimens are still moist, and they fit together tightly
enough so that if you put a little weight on top of the stack there is
little evaporative loss.  I routinely leave them over a weekend without
worrying about them drying out.    They can be written on directly with a
Sharpie which wipes off with a little 70% ethanol for reuse.   They're
cheap, too--the plates only run about a dollar each and some sources even
sell the tops separately.
Mike King
Dept. Neuroscience
University of Florida
Gainesville FL 32610-0244
352-376-1611x6499