Put cut lenghts of string trimmer line into the holes and then screw into that. Holds fairly well, moisture resistant, no drilling. The downside is that the string line will have to protrude slightly from the hole, or it will push through. The upside is that the seat base will hide it for you.
Worked for me.
Note that you will have to tighten the screws occasionally.
I spent most of my money on booze, broads and boats. The rest I wasted. - Elmore Leonard