Many people have seen the external mouth guard that I use, a few have asked about it, but I have never seen anyone try it. It is
streamlined and lasts for years. There is an excellent picture of it in the Jan. l999 issue of AQUA , the PADI Diving Society
magazine, page 60. It consists of a piece of closed cell foam, glued to the underside of the frame of the mask,  then moulded to
fit the face.  Here is how to do it:

1. Laminate high density foam, 5mm thick (for the outside), to softer foam, 10mm thick (for the inside, which is close  to mouth). Use Lepage contact cement, premium quality. An option would be to just use one thick piece of foam.  Evazote from Mountain Equipment Co-op Canada is excellent for the inside layer, or as one piece, not laminated.
2. Make a cardboard template to fit the bottom edge of the mask frame, allowing ample material below to cover the mouth. Trace this onto the foam and cut it out with a very sharp, thin knife. Some triming can be done with scissors.
3. Dip the foam into very hot water, near the boiling point, for about 10 to 20 seconds to soften the foam.
4. Quickly  put the mask on and mould the foam to fit the face.  The foam will cool quickly and become firm. The foam can be  reheated for further shaping or squeezed to make it thin and more dense.
5. Trim the foam to fit around the snorkel.
6. For best bonding of foam to mask, sand the mask frame to roughen the surface before applying contact cement.