You can make a filter baffle out of a plastic bottle. You can use a water bottle, but soda bottles are better... the plastic is more stiff and will retain its shape over time better.
1) Cut the ends off of a plastic water bottle - the remaining piece should be the same length or longer than the filter outflow.
2) Make one cut down the length of the bottle so you can spread it open.
3) Wrap one end of the cut bottle under the bottom of the outflow and the other end over the top. You'll most likely need to tape the bottle to the filter or it will come off.
This will slow the water down. If you do a really good job, it will force the water to the back of the aquarium where it will gently trickle down the glass. You might need to experiment with the angles a little, because if it's trickling out right next to the intake and making air bubbles, the air bubbles in the intake will make an annoying slurpy noise.
You can also shove some filter floss or sponge material into the bottle around the outflow... this slows the water down even more.
There's a DIY link for this on the UltimateBettas Forum... I was going to dig it up for you, but the forum seems to be down right now.