This is every bit as difficult if not more so as finding and hiring a good employee. And it sounds like you only want to rent the time that you aren’t using. That makes it even harder. You are already training in the “sweet” spot times of the day. How likely is it that you will find another trainer that also doesn’t need the space in those times? Is your studio so small that only one trainer can work at a time? There is a lot of information that you didn’t give us which would go into answering your question.
But to attempt to answer your question, I would say that you need to become acquainted with more instructors in your area. The studios near me that do this type of thing are well known to all the instructors here. Few people interested in renting space in this way haven’t already checked out these studios. And I would imagine that the usability of your studio (what equipment and the space itself) will have a lot to do with how attractive your space is to trainers. And of course, your fee.