Sometimes just bending in the flap at the back edge is enough to hold the catcher on tight .(vice grips in image)
Stronger spring usually helps.
People that can't be bothered fixing the problem just clip an elastic cord around the flap and the catcher.
Maybe get a battery clamp bolt or similar and bend to a right angle then drill a hole in the catcher and with a couple of washers and 2 lock nuts, bolt the small latch in place to stop the flap coming off then you just twist the latch for on or off. I've had this type of latch on a mower before and worked well.
