Exapanding-Lung is a respiratory training device that retails for $40. This is a similar tool made for less than $10. And can be easily adapted to fit specific mouth sizes and shapes.
Here's what you'll need:
A mouthpiece from an old snorkel or a replacement from a dive shop
A plastic valve that can open and close on a slide, such as a water flow regulator valve for sprinkler systems.
A zip tie or super glue.
Step 1: The Mouthpiece
A diver should have a few left over snorkels hanging around the dive bag. Feel free to sacrifice one of these.
Or, it is prudent to visit you dive shop and pick up a replacement mouthpiece, making sure to thoroughly sample each one for that perfect fit.
I find that the LONG BITE works great for my horse face and adequately fits my bursting jowls. Be sure to choose a good mouthpiece because you will have this in your face for at least 30 minutes a day for the rest of your diving life.
I use Diver's Supply for all my online diving needs. I suggest you do the same.
Get ready to start doing impressive things like this.
Step 2: The Valve
In our case, the valve should be plastic. The lighter the better. And make sure one end is relatively circular. I browsed the hardware store for nearly 2 hours fitting all types of valves to the pictured mouthpiece and finally settled on the hose adapter. Its light, sturdy and cheap: about 3 bucks.