A first plate comprises a first lip, lip gaps, and first indexing formations. The first lip extends around a portion of the first plate. The lip gaps are distributed along the first lip. The first indexing formations are distributed along the first lip between lip gaps. A second plate comprises mounting points, a second lip, mounting tabs, and second indexing formations. The mounting points may receive mounting fasteners to secure the second plate to an object. The second lip extends around a portion of the second plate. The mounting tabs are distributed along the second lip and positioned to align with lip gaps. The second indexing formations are disposed on mounting tabs to engage first indexing formations in response to the second plate having mounting tabs inserted through lip gaps and rotated until at least one first indexing formation is aligned with at least one second indexing tab.