Recommend placing a hook point (at least) at each corner. Once you tension two STAS clips on a side, they will not easily come back out of the insert's hole.
Suggest further anchoring the bottom with something like a command strip mount, ideally one that adds a bit more of a gap behind, to compensate for the rail hanging gap; e.g. https://www.printables.com/model/330094-3m-command-strip-mounting-for-hsw
The author remixed this model.