Evans added a penalty and two conversions for good measure and regional director of rugby Darryl Hill said, "We are absolutely delighted for the boys, they have worked so hard this season and now they have something to aim for at the end of it."
The Ospreys, who have been outstanding for much of the season, scored one try through Liam Osowicz with Daniel Biggar dropping a goal and Lee Halfpenny kicking a brace of penalties.
In the other game, Blues North scored a first win on the season - they defeated Newport Gwent Dragons 33-17 at Aberdare.
Gavin Lucas, Jake Thomas, Mike Powell, Mark Fleet, Tom Price and Adam John bagged tries for the North with Nathan Rees kicking a penalty. The Dragons ran in tries through Spencer Gibson and Nigel Jones, but their contribution and seven points from the boot of impressive outside half was not enough to prevent the Blues from winning for the first time.