One at the bottom isn't really needed since the moment you press even just a small bit in, the hole you're pressing into acts like a clamp.

And TBH, clamping the top is an overkill but one of those things where it's like... so easy, might as well do it? If the shell is going to break, it's going to break with or without a clamp. That one above that shattered had a clamp on the top up until the very last millimeter I could keep it on.