As terra_t outlines, introduce a "Reservation" resource which contains 1 or more slots, allowing someone to (attempt to) create a Reservation containing several

As terra_t outlines, introduce a "Reservation" resource which contains 1 or more slots, allowing someone to (attempt to) create a Reservation containing several 15-minute slots in a single request. If "N 15 minute slots" are something that you want to either all suceed or all fail at the same time, then what you really have in this concept is another entity/resource to add to your model.