There lots of opinions on how to go about this type of repair. Here's a thread where I asked a similar question, and got lots of good answers:
Fiberglass/plywood floor repair questions ...
In my case, I purchased bondo-brand fiberglass resin (which is a two epoxy), some fiberglass cloth at a local harbor frieght. Any autosupply place will have this stuff too.
To do the repair:
* cut a cardboard template to make sure it will fit right. Then cut a piece of plywood to match the template.
*Prepare the resin with the hardener, then using a disposable, cheap paint
brush, to cover the replacement plywood (both sides). I did this before installation.
* Surface prep: sanded/wirebrush the surrounding surfaces. You must remove any paint
. Resin will bond with exposed fiberglass.
* Installation: Prepare more resin; put the board in, cover the board and the surrounding area with resin. Gingerly place the glass cloth over the board and edges, then carefully add resin on top. This was surprisingly fussy, as the cloth will come apart if you touch it after it meets the resin.
I wore sugical latex gloves. These allow you to use your fingers to smush the resin as needed.
Use something to apply force to bring two surfaces together. You can use screws, then take them out when you are done, then bondo the holes.