Remove the seat and temporarily clamp the seat re-inforcement piece to the underside of the front edge, with a little bit extending past each end. Mark the top cut line on the re-inforcement piece (where it extends past the edge of the seat), and remove the re-inforcement piece. Mark the front cut angle by lining up the bevel gauge (still with the vertical angle that you copied in the last step) with the top cut line, and cut the re-inforcement piece along these lines.
Test the fit of the seat and re-inforcement piece, and when everything fits properly remove the seat and coat the top edge of the frame with several coats of un-thickened epoxy. When it's well saturated, brush on a layer of adhesive mixture and replace the seat. Tack the seat edge to the frame with several small nails to temporarily hold it in place until the glue sets, and fillet and glass the seat to the sides and frame. Install the re-inforcement piece with adhesive mixture and clamp until cured.