Cayennita,
You're the MAN! (Or woman, can't tell). Never use any type of urethane sealant on polycarbonate. It WILL craze because of it. Also, anything with ammonia (Windex) will craze and cloud plastic of all types. Good ol' dish washing soap is best.
If there is a lip to install this on, consider 3M VHB tape. Adhere the light with that, about 1/4" back from the edges. Mask the glazing and wherever you don't want to get sealant. Run a nice bead of DOW 995 in that gap and TOOL it smooth. Sealant must be tooled to be water tight. Anybody that tells you differently is speak a dialect of testosterone. Home Depot sells a caulking smoother the does a good job of liquefying the outer surface of the joint and really smoothing it out. Pull the tape almost immediately. The result is a bead that looks like a perfect rubber gasket.
I work for a commercial glazing company.