This. It's extremely difficult to hire well, because no one knows what that even is. Firing well is somewhat easier because almost everybody knows who must be fired, but taking action is the difficult part.
Once proven that a candidate is able to learn and adapt to the job, they should be moved to a full-time position, with pay above what is currently being given to that role. If they aren't excellent, let them go. As a company you want to advertise this promotion experience so that people who know they will pass this test will feel more comfortable signing up for a "lowly" temp position.
The top 5% or so of candidates might not take that offer, but you can leave a hole for exceptional candidates based on past experiences and proven success.