[Mr V]

You are on the right track with either measuring the weight or density. Both of those sensors can be located outside of the bin so they aren't affected by the dust in motion or a static charge building up a film of dust on the sensor's lens.

The easiest would likely be the density sensor since you can buy a complete ready to do solution relatively cheaply, though it might not be effective if the bin is made of metal.

I'd recommend the Franklin ProSensor 710 since it will act as a bar graph and show you as your are approaching full before you get there. A little modification to it would be best, replace the momentary contact switch with a standard on-off switch and wire a battery eliminator in so you don't have to replace the batteries all the time.

If you want to go a little cheaper then you could go with a basic stud finder which can be had for under $20 though I'd still recommend the switch and power source modification