The fence
a building
I think the border of the city or like a fence or wall would be the best one I can think of.
It could be a gate. It is a gate because the door is the cell membrane. See you need to get past the cell wall before the cell membrane.
The Fence because it keeps everything safe from the surrounding environment.
You can relate a cell membrane to the school's office because they control who comes into the school. (i.e.: visitor's have to check in at the front office they can't just walk in without checking in)
A prokaryote is a kingdom but it fits into into the Monera kingdom. Prokaryotes are molecules surrounded by membrane and cell wall.
A fence around a building it keeps bad stuff and the other stuff in
A nickname for the cell wall is the barrior. It is also called "bricks for building" because it protects the cell and helps it maintain its shape.
A castle. hard outer wall, important nobles inside. protected by knights, etc.
Anything that is somewhat rigid and provides structural support, and is osmotic (can passively allow small molecules and water through). An analogous comparison is one that allows someone to relate a likeness to something from something completely different. There are multiple things that can be analogous to a cell wall. A filter. Or maybe a screen door.
the wall would be a cell wall. the door would be the cell membrane and the mailbox would be the Golgi bodys.
The cell wall of a cell would relate to the body/exterior of the car