If you don't fancy siliconing your screen freehand you could cut a mold out of cardboard, line it with clingfilm, put silicone in the groove and flatten it with the clingfilmed surface of a book.
I did something similar when I needed a quick custom seal between a glass jar and glass lid, I only used one bit of clingfilm to stop the lid sticking to the jar during molding meaning the seal is stuck to the lid but it might work for what you need.
