I have bettas in their own 5 gallon tanks. The filtration I use is a sponge filter, run by an airpump, and a gang valve to control the airflow and water movement. There's a pic in my gallery. But my bettas don't make bubblenests either. One of them does a little bit, but not much. I miss it too!
I haven't tried this, but you could cut a plastic disposable cup in half up and down (don't make the cup shorter, cut so that it won't hold water anymore).
Put the concave side down, just above the water surface, and fasten the bottom of the cup to the side of the tank. The water surface disruption should stop under the cup, and the betta may go under there to make a nest.