Just glass straight from the edge of the gunnel across the void to the transom. It should span it fine.
Grind out a v in the splash well join before you glass the two bits back together. To keep it all in shape, screw timber to the underside with screws in both pieces to keep it flat. Put tape or plastic on the timber to make sure you dont glue it in as well.
1st photo, glued together with a butt joint, screwed down holding everything flat.
2nd photo, screws out and reground ready to glass.
3rd photo, glassed the v back together.
Just my opinion. Use it at your own risk.