Hi, my name is Kerry
Clean out all of the rotten wood down to the hull.
Make sure the area is clean and dry, you can use acetone to remove any traces of wax or oil.
Wet the area down with resin and apply a layer of fibreglass matt and saturate it with resin.
Apply the first layer of coosa board and press it firmly in place until the resin kicks.
Trim any sharp edges of matt that may be sticking out and fill any voids with fibreglass filler, the type with fibres in it.
Apply another layer of matt and the second coosa board.
After the second layer hardens, trim and fill again, then cover the whole works with another layer of matt overlapping onto the surrounding area.
I found this on a web site;
You do not have to encapsulate a Coosa panel in fiber-glass and resin, as would typically be the case with plywood, to prevent water absorption; however, glassing the panel adds stiffness and strength. Therefore, if the original equipment manufacturer encapsulated their plywood panel for structural reasons, you will want to do the same or find another method to use to compensate for the loss of external strength derived from the resin and fiberglass skins.
I'll be happy to answer any more questions you have about this process.