Sounds like some ply over the whole area is best.
I use West Epoxy system, widely available from yacht chandlers and east coast fibreglass. There is lots of information on the West web site on how to use it. Check out youtube videos as well.
I usually buy dispensing pumps as well – it makes it easy to dispense the right quantities of resin and hardener.
Filler is also needed – there is a selection guide on the West web site. And disposable gloves.
Some means of clamping the timber in place will be needed, perhaps wedges against the other side of the stern tank.