I made 15 of these for my killie tanks they work very well
i made them from glass they are 5cm x 5cm x 12cm tall i drilled lots of little holes at the bottom 4cm of a 13cm long 20mm pvc pipe then put that in the glass box then put stone/gravel in up to 5cm, then filled it up with filter wool stuck and airstone down to the bottom of the pvc pipe put it in the tank turned the air pump on and away it went.
i could see things that were floating around in the tanks being sucked down onto the filter wool. It does a great job IMO
see diagram
HTH