"escape hatch plastic cover"
And you do mean "escape" hatch and not skylight correct???
If you mean "escape" hatch see
http://www.amazon.com/Hengs-68631-2-Elixir-Escape-Garnish/dp/B0027XK788Didn't see cover only, but manufacture name might help you tracking down part.
Shoot me PM if you strike out the finding part your looking for. I was looking at one couple weeks ago. Thinking about putting in Class C above the cab over. (And was trying to see if my fat body would fit thru it.)