I am working on a couple of possibilities for these, but there are some serious limitations due to the design/shape of the rear of the car, the hatch opening, and inside of the trunk area. If folks are willing to have a bracket that is screwed in to the sidewall from below, that is very possible. I don't like designs that force folks to drill holes in things and don't believe that many folks want to do this, but it is something that was considered and could be done.
The brackets aren't actually necessary if you do not want to use them. Without them, when the hatch was closed, the tail end of the ModelYShelf would simply fall on top of anything you had in the trunk and cover it. If nothing was in the trunk, the tail end would rest on the floor. The brackets would still be included with the kit, you can simply choose not to install them.
HTH,
Steve