Is there an easy way to trace rocker templates onto the rails after the outline has been cut? Seems difficult when trying to duplicate a board's rocker.
No, there isn’t.
Put your rocker template along the stringer. shape the stringer area to the correct rocker.
After making sure the blank is level from rail to rail, (not nose to tail) place a 2’ level perpendicular to the stringer. Shape the rail area to match the stringer ( or higher for concave, lower for Vee.)
At least that’s my method for exact rocker.