The plywood idea sounds great. I wouldn't use a drop cloth or tarp because chickens are always scratching around and catching things in their toesnails. I would suggest going to a fabric store or craft store where they sell rolls of the stiffest heaviest gauge clear vinyl and buy a yard or two. I get this heaviest vinyl because it doesn't buckle up or wrinkle or fold over itself - a screw or nail in each corner to attach the vinyl to a slide out tray would also work. Another idea are those plastic floor runners that are about 20 inches wide and about 6 feet long that people use to cover floors where there's heavy foot traffic - it could easily be cut to size with heavy-duty shears and it's sturdy and something the chickens can't move around from all their scratching - dropcloth or tarp would be too flimsy. I've also seen people build their own litter tray out of scrap wood with one or two inch tall sides to keep the litter in the tray and then place it under the perch - a drop lid can be cut into the side of the coop where the PDZ tray can be easily accessed for sliding out for cleaning. You have all sorts of possibilities. We search our neighborhood on trash days to find scrap wood and stuff to use for the chickens. All our low shelters in the free-range yard are made out of salvaged headboards, broken tables, cupboards, etc. Both the doghouses that our chickens use outside for shelters were recycled or donated to us.