Is your background part of your sealing solution? My guess would be that whatever the background is made of, it isn't water-tight. I have successfully (and quite easily) sealed glass with plexi using silicone, so that shouldn't be your issue.
As for the plant, I would add items horizontally to increase the upstairs real estate, not vertical from the bottom. I prefer magnetic furniture over suction cups, though my supplier is running very short on them these days.