Your best option is to take off the ziplock bag - that's not going to hold up. Get a piece of eggshell and place it over the hole. Then use a marine epoxy putty to cover the hole and the eggshell completely. I've used Aquamend with great success. You can get it at any hardware store.
You'll want to knead the putty until it's a solid color with no marbling, then knead or roll it out into a thin sheet. Cover the eggshell, which should be covering the hole, don't worry about covering too much of the snail's shell itself. Be sure the epoxy is firmly attached to the snail's shell, then put the snail in a shallow dish of water until the epoxy is cured. The epoxy will dry to a hard, rock-like finish and will permanently seal that hole.
It's very important that you do this IMMEDIATELY, as the hole is right over the snail's lung. If you do not seal up the hole, the snail could puncture its lung and drown.
