This is what I would do on the shelf, take it for what it's worth.
I would create two different dams/beads on the shelf. One around your topoff incase you are or decide to use some type of kalk in it. I would dam/bead that section so that it lands into the bottom floor. Don't want to flood your tank with a couple of gallons of kalk water.
The rest of the area, I would dam it so it goes back into the tank. It might collect a little dust and dirt on the way but if you do get a small leak, it should help keep the system going for a while longer. I would also try to do something like that on the very top if you can. Try to give all possible water a place to go.
Again, take it for what it's worth.
Brian