Here is an image of one. Yes, it just presses right on the opening (is that the stoma?!!)
Like the inner canula, it does hinder breathing, but not as bad as the canula.
When the air is dry, I put one on at bedtime. Usually, around 1:00 a.m. or 2:00 a.m., I remove it. At that point, the humidity is created will cause the mucus to greatly loosen for an easy expel. That is good, because at that time of night, who wants to be cleaning out a tube?
Let me know how it works out for you if you decide to get some.