Don't, I repeat don't use screws in the fiberglass unless there is wood or metal imbedded in it for the screws to grip. They tend to shatter the gel coat and damage the glass fibers and the next thing you know the pull out and you end up with a bigger hole.
Use either alum. pop rivets (Lord strike me down for saying that) or use SS bolts and nuts.
Do not use SS pop rivets as they are very hard to pull and create too much crush on the material.
Also they have a steel mandrel which will rust and then leave stains on you paint
or gel coat thus defeating their purpose.
I have installed 1000's of SS, alum., steel and Avex, expanding, blind and air hammered rivets.
I can only pull no more than 2 dozen SS rivets with a hand rivet gun and I have to quit as my hand is so sore.
When ever possible when using rivets on FG use a alum. back-up washer. If there is a metal fitting then they are not required.
If you don't, the mandrel expands the rivet on the back of the FG and partly into the hole again causing damage to the glass fibers. Eventually they will fail because the fibers have been weakened.
Hope this gives you some guidance.