Okay, there are two ways of working with this, you could get an object that represents the nucleolus in shape, or a representation of its use.
If you want something that is similar in shape, maybe you could try a small bouncy ball as the nucleolus is a little squidgy so that could work well. I would say a marble, but as that is solid, maybe it wouldn't be so good - totally up to you though obviously.
However, if you are going on functionality, you could perhaps use a USB drive because a nucleolus is where ribosomes are assembled and as you know, a ribosome is a large and complex machine.
Therefore, my thinking is that the nucleolus is serving this machine, possibly in the way a computer drive would. A router would be another idea you could use; I'm trying to think outside the box a little here :-)
Something else just occurred to me when I looked at the diagram of the nucleolus that I have included above; it looks like a plasma lamp! I don't suppose you have one of those but if so, it is another thought.
Hopefully, that has helped or at least given you a little food for thought, let me know what you decide!