you put one of these {img]
http://www.bencere.co.uk/images/bensink.jpg[/img] in a drill, to drill a hole the size of the screw head so it will sit lower than the top of your plexyglass.
step by step:
1. Decide on size screws.
2.Find a regular drill bit that will drill the diameter of the screw
3. Drill your holes
4. using the countersinking bit that you go out and buy, drill the top of each of the holes just enough that the head of the screw will be lower than the top of your plexy, but not too much because then they just rip through.
5. pray for a freak snow storm in july.