I've seen a lot of posts talking about the problem with the filter current pushing the betta's around. I also had a few comments about the little water bottle trick that I used on Taco's filter. I've taken photo's and I'll go through the process step by step. This can be modified to work on all internal filters (such as Whisper filters) and regular hang-on-back filters.
A round plastic water bottle and a pair of scissors.
Take a regular, round water bottle, and cut it in half. Throw the top half (the part with the lid) away, you'll only need the bottom half...
Take a pair of scissors and cut a slightly wide slit 3/4 way across the bottle.
You want the opening to fit across the outflow of your filter...
The closed end of the bottle should be facing away from the nearest wall of the tank. At the top of the long slit, on the side closest to the closed end of the bottle, make a small slit. This will help position the bottle into place when it's set on the filter.
Position the bottle over the filter outflow. If it doesn't stay in place, make the small slit (as seen in step 3) a little longer.
All done! The water current will now flow out of the open end of the bottle, and against the tank wall. This will result in very little surface agitation and current, and make a more peaceful environment for your betta, while still allowing a filter to be in place.
Easy peasy! 8)