These food grade silicone stoppers feature several vent holes lightly sealed with a silicone top flap. Gases can escape, but cannot get in. Just put the stopper on your carboy, and forget it. No worries about evaporation or water getting into your fermenter. This #10 size is pictured at right above, and fits all Fermonster lids, and other containers with a 1.75" opening. This is 1 5/8" diameter at the bottom, and 1 7/8" diameter at the top (right below the lip).
Click 
here for pdf instructions on how to disassemble and clean this product.
You can be the first to ask a questions.
Works well but...
Those who have problems with them are washing or sanitizing the stopper before insertion. I always wait until the stopper has dried off and that the neck of the carboy is dry, if they are wet, yes, it tends to pop off, They are made of silicone and need to be dry.
stopper
Have not used it yet, but looks like a cool idea.
First Use report (so far)
I hesitated giving a numerical rating yet (but is required). I wanted anyone else that has just gotten one of these to BE CERTAIN that it is firmly seated in the carboy! After going through the process of boiling and all, making sure to keep things clean and sterile, we set the carboy aside with the new airlock, only to find it on the floor come morning. It evidently built up pressure enough to dislodge the stopper before opening the rubber valve. We're now waiting to see if this batch was compromised with airborne particles. Fingers are crossed. We FIRMLY reseated this time and it's stayed. Obviously it takes more pressure to open the rubber valve than to blow a bubble through water. I like not having to deal with the old style water filled airlock. *Update! Have not had another problem. Works well.
OK to use with Plastic Carboys
I agree with the other reviewers in that I would *not* use this with a glass carboy since it is difficult to get it securely snug on a glass carboy. It works fine with my Better Bottle but I limit my use of it to secondary fermentations.
Good for Better Bottles
I use the stopper inmy better bottle, but woul dnot use it otherwise. I like it because when moving a Better Bottle the bottle will distort making it easy to suck in the airlock fluid. With this stopper that is not a problem, but it does seem to allow more pressure to build up inside the Carboy. I probally would not use it in a glass carboy, just in the Better Bottle, and even then only after active fermentation is over.