Hi @Jann3,
I think many of the issues you see here on the forum are mostly from folks who may not have communicated clearly from the getgo, so kudos to you for getting ahead of this. I would put the checkin/out times in your House Rules, response to inquiry email and welcome email with an additional note that checkin/out is an automated system with a timed pin controlled lock... Also be prepared with a friendly "Sorry Charlie" email when someone asks to checkin at 9am :).
So you don't have to keep typing emails you can either have a master word document that you cut and paste from or copy and paste from previous messages in your box.
Good luck