How to pick desired reservation days spesificly rather than having all days open?

The main booking for the business is handles via phone and direct contact with customers (and the client want to keep this routine).

The client want to use the plugin to advertise spare time left over for a cheaper fee, thus only requiring maybe 3-4 hours a week. The only way of making this plugin work would be to make a lot of fake bookings to block off the time leaving the desired open reservations available.

A reversed view would be awesome, actually marking the work hours spesifically instead of using generic ranges as templates for the whole year. Do you know how to achieve this? or any other plugin that does this?