Simply rotate the level slightly to insert it in the top mount.  Then lower it into the bottom mount.  Reverse this sequence to get it out.

Attach with your chosen method.  If you need drywall anchors, try these.

[https://www.printables.com/model/176084-6-screw-drywall-anchor-with-threads](https://www.printables.com/model/176084-6-screw-drywall-anchor-with-threads)

No support required.  Recommend 3 perimeters for rigidity.