i have a flat down box that i made out of pressure treated wood. I use butchers wax which is this really thick rub on wax that soaks into the wood and makes it really dense and slippery. it works really well but you have to wax it a couple times before you session it for a while. once its all waxed up its perfect and as long as you edges are dull you wont have a big catching problem.
me and a friend recently built a flatbox and were going to use mica but were sugegsted not to because it is extremely sharp and will chip so we ended up using wood and i threw some reg candle wax on the night before and it slides as good or not better than a normal plexaglass box