Pre-cut wood donut. Drill a hole that has a diameter that is 1/4" larger than the rosette. Then glue this donut on the underside of the soundboard to create a small ledge on which you can glue your rosette.
The sizing corresponds to the rosette size. Ex. If you are installing a small rosette you should get a small donut. If you are installing a jelly-filled rosette you should get a jelly-filled donut, etc.