I did a writeup on it a while back somewhere, I'll see if I can find it.
It's pretty simple, just a 4 or 5 inch piece of 1/2" pvc
and a piece of aquaclear sponge filter insert cut into a square (the ac110 filters are perfect for this.
You can drill holes in the end of the PVC
if you want, but you don't necessarily have to.
I cut a small hole in the filter sponge so that I can slide the PVC
tube down into the center of it. I also use a drill and put an angled hole near the top of the PVC
tube to slide the airline into. If you drill it at an angle it helps keep the piece of airline in place without having to secure it.
As far as weighting them goes, siliconing a piece of slate works, but i have noticed if you squeeze all of the air out of the sponge it will eventually sink on its own.