I recently came across a rather cool idea that solves this problem. I ended up using it on my tank too, since the incoming water from the filter ends up excavating a giant hole in my sand as well as annoying my betta.
Filter Currents: A Betta's Worst Nightmare
What you do is cut out the middle of a plastic pop bottle, and then cut that in half lengthwise, leaving you with a curved piece of plastic that you can attach on top of the filter (though for me the curve of the plastic itself holds it in place). The water hits the curve and gets majorly slowed down, leaving you with very little current.
It works great!