What Ben said. All Roombas including and following the 700 series have bin-full sensors that use an IR emitter and sensor that goes across the mouth of the bin.
To provide a little more detail, research a material's 'IR Transparency'. Most clear plastics and glasses are transparent to infrared, but not all. Even some opaque plastics are IR transparent, and this can make your life a little easier by making sure the light picked up by the sensor is only the IR you're shooting at it. You'll need to double check whichever materials you wish to use, and make sure that the IR beam can penetrate.
- Should be able to penetrate just find, as long as you pick the right material
- 25cm shouldn't be an issue, but check the specs of the sensor and verify it against the log output from the roomba
- Mostly to protect the sensor from debris
- Mostly for mechanical reasons. It's at that angle in the 800 series roomba to make it easier to insert and remove the bin with one hand