I fabricate with 3/8" cell cast plexi all the way down to 1/16" extruded plexi and you don't want to manage the upkeep for a display case.
Precut lexan is the answer because the likely-hood of you cutting it properly out of the chute is next to nil, you'll chip it - takes special drill bits and blades to cut all polycarb and acrylic sheet.
This company has never gotten an order wrong for me and I've orded up to 2K worth of cell cast at a time, they'll cut it to your spec, perfectly. You can find it a little bit cheaper other places but you may get crap, molded backing, scratches etc. Modern does it exactly right every time and with this stuff there is no margin for error. Also watch prices, acrylics fluctuate like commodities as the price paid for bulk is tied to the cost of oil. Sometimes waiting is smart, sometimes buying now and stocking up.
This is one of my sculptures made with MP cell cast acrylic, with the right blades it cuts like butter with no melting or burrs, but it scratches so easily it has few practical uses where it'll be touched.