Hi Kevin, if you have access to a sheet metal brake you could add one inch on each side of your Lexan and notch the corners and then bend the Lexan 90 degrees to fit over the remaining skylight.....if you try this, set back the clamping part of the brake about 1/4" so that you get a decent radius on the four bends.....if the corners are bent too sharp it could damage the Lexan....alternatively you could bend it in small increments of a few narrow
brakes to achieve the desired radius....that will work also....assume that you`d use about 1/8" Lexan.....doing the mod this way, you`d only have to seal the corners against water leakage and use silicone to cement the Lexan to your dome around the one inch wide section and where the Lexan contacts the flat part of the remaining dome....I also wouldn`t worry about water run off since it`ll be stored in a garage and while driving water should be blown off or run off when you start to open the vent.....good luck...
......Benny