The three leaning rocks on the left side might remain in position longer if the flat rock you're bracing them with has a side touching the glass. That would stop it from sliding away from the leaning rocks over time.
Alternately, a small flat rock between the glass and the bracing rock to fill the gap (and avoid a point being pushed against the glass) would hold the structure without having to move the group over to the right.
Eheim Pimp #459 - 2213, 2234
SunSun Pimp #47 - (Perfect) HW302