I am not sure a secondary latch is needed if you modify the latch stricker plate.  The latch fingers will have greater overlap.  I added an extra plate with the hole enlarge to accept the delran piece - see attached.   I used countersunk screws for clearance.   As a way to deal with pilot error, I added a canopy latch switch which illuminates when the latch is open and if open and airspeed is above 30 kts, I get an aural warning.