You would have to make the center tube wide enough to take an airstone - you then drop the airstone (attached to airline) to the bottom of the tube and this then draws the water through your filter
Yeah that would work. A small powerhead would be far more effective but an airstone would work just fine. Just a tube with no stone would be pretty useless I'd have thought.
to be honest though with the price of a Huey Hung sponge filter - I paid £6 for a single foam and £8 for a double foam - I cant see the point in making one.
Plus a water bottle stuck at the back of the tank is not going to look great is it.
(If you want to try this or whatever ) I made a simple addition to my hob filter (Hang on Back Filter), Simply buy some foam and then cut a cube-like section and cut a hole about half way through it and place it on the end of the intake tube for the hob filter. Yes it does require cleaning about once a week but it will save a few fry lives