All these replies are good, I like the skylight idea its simple, I had not thought of that one...The trick to large hole replacement repair is a plastic sheet you lay out on a flat surface like a table, this type of plastic sheet is
sold for this purpose, you lay out the
fiberglass to the number of layers needed wetting between the layers resin/hardner as you lay up the fiberglass on the plastic...When cured you peel the plastic off, put the piece to the hole and draw the square to fill it, make sure you have enough fiberglass to cut a second square again only larger than the hole this time... put the smaller square on top of the larger, using epoxy products put these patch forms in place and a prop to the floor holding it until it sets...sanding and sealing the patch comes next...there are do it your self dvds I can suggest that shows the whole process...its the same as boat hull repair. The finish coats are another process altogether, being familiar with the products well first will make the hole go poof...But the cost of doing it yourself often is as much as having it done by a shop...less fun for me...but about the same in hard earned cash.
Harry