Ken and I were talking today about another probable solution.  It draws me back to the frame I initially had planned for the back of the window with a recess to fill with Dow 795.  That would forego a liner of any kind.  It surely would validate my faith in Permadri products.  We'll see how this free liner pans out.  I do have some concerns about vinyl since historically it isn't fish friendly and doesn't handle a bio load very well.  Clear plastic products yellow very easily under the same conditions as well.  I'm willing to give it a shot though.