Further developments of this type of model usually involve quite a bit of guess work and experimentation. Should the tongue extend out of the mouth? (Turns out- yes) Will making the tongue heavier by adding extra layers of card make the rasp louder. (No)
Further experiments involved making the tube as short as possible so that it fits inside and model. I got it down to 30mm.
I've also worked out a way of fitting the whole assembly to a tube with the air coming in at 90 degrees. I'm not sure if I'll be using bellows or whether it will be a model that you blow into. Angled tubing gives me the option to choose. How does the rasp work? My theory is that with the tongue at the bottom of the mouth. (2nd picture) the air blows over the tongue, the fast moving air causes a reduction in air pressure causing the tongue to lift. As it lifts, the air flow is redirected under the tongue making it move the other way. The same process is repeated over and over making the rasping sound.
The picture, right shows the latest stage of the rasp. Blow through the top to make a suprisingly loud rasp!