Kerik --
I made my own years ago from plywood and cedar.  They are still in use.  I
painted and sealed the inside with floor enamel.  I would now recommend an
epoxy based paint.  I would encourage you to make them large enough to
comfortably accomodate a 23 x 31 " sheet.  The least expensive way to make
large trays is to make a frame the size you need and drape the whole thing
with plastic.  Throw out the plastic once in a while.
Greg Shattenberg
Shattenb@megalink.net