just to add my forpenneth to the considered opinion above . . . .
I think there is too much made of these devices giving no resistance - I think they shoud have resistance which woud mean that if you can play a piece with the device in you bell it becomes much easierto play with it out.
What a pile of ****! (self-edit)
What's wrong with a DW practice mute? A good bit of resistance to push diaphragm against, tremendous stuff. If it's too loud, blu-tack one of the whole, like what my brother does, nutter!
Amazingly this is not a spoof. They do actually exist and I had a go on one a while back. The mute itself is well made and robust and very effective. Its quieter than a denis wick practice mute and offers about the same or a little less resistance. Unfortunately the person who owned it didnt have the stethoscope bit with them so I can't comment on how good the whole ensembe is.
If I needed a cornet practice mute I might be tempted to order one. They are quite cheap after all.