I remember asking Richard if it was possible to make a Zero Plot bigger. The answer is yes, but, you cant' throw dirt at the edge and expect it to stick. what you have to do is dig a trench through to the bottom of the plot, and somehow push the two parts apart.
The best place to try this is at the edges of the plot, as that's where it's the thinnest. You need to keep the edge connected to the plot, so perhaps some sort of scaffolding or a beam secured across the trench. In fact, if the beam is hooked up to a piston of some sort, it could be the thing that pushes the two sections apart.
You can't fill the trench up with dirt, as it will just fall through. You have to wait until the plot lands somewhere, and then get that bobcat going and fill that trench up with new dirt. When the plot leaves, it takes the dirt with it. Rinse and repeat.
Unless you compact the dirt as you put it in, it will be loose soil and can dribble away. So compact it while your on a world. Or.. fill the bottom of the trench with concrete. That lets you compact the soil in your leisure.
How big can you make it? I had my players hear from some survivors of a plot nearly a mile across. Think about that for a second.