duh....despite the name of this thread, my brain thought it was wider and those pieces were both going on the back wall. i am such a ditz sometimes....LOL and if your background pieces are going to be siliconed directly on top of the edges of the egg crate, then you really won't need to silicone them in. the rock wall will keep it from going anywhere, but better be safe then sorry. i had to silicone mine down in the paludarium because all the land masses were floating!!!!LOL