Acrylic or Plexiglas (same thing) should work for your enclosure provided you use thick enough material and are able to seal all the joints fully, which can be tough. Velodyne makes sub enclosures out of the stuff. I believe sheets come at least 1 1/4 inch thick. You probably would want to use at least 3/4 inch, but it's rather expensive. ~$450 for a 4'x8' sheet, double for 1 1/4" material.

Stu