anytime i cut holes in plastic i use a hole saw. (you don't need a bi-metal one made for cutting metal, just a cheapo one)
first i use the drill in forward to get the pilot in, then i reverse the teeth so they don't catch and crack the plastic.
if drilling a lot of stuff be sure to use a corded drill.
as for sealing things (if needed) i use black RTV silicone sealant