Replaced my windows with new lexan about 7 years ago. Used GE Silcone II. Found that most good silcones will work. The secret to any good seal is to sand the two surfaces no matter if it is plastic, fiberglass, or metal. Use GoJo or another hand cleaner to clean up the excess silcone before it hardens.
Had a winch that I installed with silcone II and I needed to remove after about 4 years and I had to pry it off with putty knives and chisels. Could have used the winch without the bolts in it.
