KenK:

You may be able to make the snap less likely to come open if you try the following.

If you have a punch with a rounded tip slightly smaller than the lip on the male portion of the snap, but bigger than the center hole of the male portion of the snap, give the male portion of the snap a couple of hits to try and splay the lip out a little bit. You can also use a small mechanics socket for the same purpose, but hold the socket with a pliers to protect your fingers. Don't hit it too hard, or the lip may splay so far as to make mating with the female portion of the snap impossible (The female part is usually on the flap and has a tensioned spring clip in it.).

Good luck!

Bountyhunter