tricky shape

I recently got this glass jar with an unusual shape (see picture) and I’d love to turn it into a terrarium. The issue is, the jar is quite wide at the top and narrows down, so I’m worried about the soil sliding or shifting over time, especially when watering.

Has anyone built a terrarium in a similar jar? How did you manage the soil placement? Did you use any tricks to keep it stable, like layers of rocks, mesh, or specific types of soil? Any advice on plant choices that would work well in this shape?

all tips are welcome! Here’s a side view of the jar