Lots of betta keepers use a new
soap dish (that has never seen soap) and will fasten that to the
inside of the tank wall with suction cups. The filter water flows out
and is difused by the soap dish lessoning the current.
From:

WOW

What an excellent idea. I'm wanting to add a power filter to my betta's tank
and man.... Just wow. Thank you. LOL.

Jessica
-Yet another baffled victim of a fish that decided to go MIA.