plywood is not really a great material for monitors unless you really seal it well.
lets look at cage sizes first..
for a fully grown sav, your looking at a minimum of 8'l x 4'w x 4'h
for one ackie, your looking at a minimum of 4'l x 2'w x3'h, for two you'd want to go at least 6' long.
now there's substrate, you'll need a good mix of unfertilized potting soil and sand at at least 1'-2' deep for an ackie, deeper for a sav.
i don't know much about sealant, but it will definitely need good sealing to keep the cage safe from damage due to high humidity.