Loxone sells its own 7" ceiling speaker (link) which you can and should install into the special back box (link). 

The problem is that if you plan to make only preparations for the speakers and not actually buy them, you end up with a 7" hole in the ceiling because the speaker's cover attaches to the speaker itself with magnets.

Therefore I've designed this gadget that acts as a temporary fake “speaker” and you can attach the cover to it.

You will need 6 magnets 3 mm in height and 8 mm in diameter that you must glue in prepared holes.

To attach the gadget to the back box use countersunk self-tapping screws with a diameter of around 4 mm.

Printed with PETG and 0.2mm layer.

Bottom.stl requires some minor supports. See 3mf file for details.