DIY sponge filter - how much sponge?

I want to make small sponge filters for a couple of two gallon betta tanks. They are planted, but I think they could do with a bit of filtration and some gentle water movement. I found these directions but I will use an airlift rather than the powerhead. http://www.hagblomfoto.com/article_spongefilter.htm

I got some 1/2" rigid tubing and the airline. How much sponge do I need for one betta? Would a chunk about 1" square and 4 or 5" tall do the trick? Also, is open cell filter sponge even the best material or should I be considering using some sort of matting? I'd like to make something small, simple and low-maintenance.

Thanks for any ideas.

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