The détente will generally deter accidental displacement; if necessary an aluminium or PVC extrusion, or even a small block may be screwed into the 15mm ‘drop’ gap above the panel, to stop it being lifted.
By orienting the Fixes at 90 degrees, panels can be slid in sideways. Adjacent panels can also be mounted similarly, but of course the first panel(s) installed will effectively be locked in place, unless the subsequent panels are removed: This can be advantageous for providing security for a run of panels, requiring only the last panel installed to be secure.