Cone, WOW! Thanks for the warning (and the story!). I had a glove tear out on me when I was coating some car parts with Por15, wasn't a tattoo, but I had black freckles on my hands for almost a year. Hard to explain to patients sometimes...
What if I were to fiberglass the "sill" then bed the Lexan onto it with butyl tape and some kind of trim (aluminum maybe)?
Hmmmm, maybe some testing is in order...
GPW, I like that plan, simple and fairly easy to execute. I've seen Aluminum C-channel at the hardware store that could be pressed into service and keep the wood quotient down as well. Do you think a simple foam rubber seal between the two pieces would be weather tight enough?