Here are Kim's instructions for a baffle. I searched for a diy for one but couldn't find it. If you have any questions, I'm sure Kim will be glad to help you. :)
"I have a baffle
I built out of a plastic water bottle. Just cut the ends off, then cut the remaining tube lenghtwise. Now you have a curved piece of plastic. Attach one end to the filter
cover and stretch the other end so that it curls around the outflow. This diffuses the current to the back of the tank and also cuts down on the power of the outflow a LOT. In the tank that I used it on there is no current at the top, and the little current that is left is so weak that my fish will play in it :) If you can't understand how to do it from these instructions I can post a pic."