Lego!

[kresty]

In case it wasn't clear, here's how four of the bottom sections are supposed to fit:

Bottom Greeble Fit

My earlier line sketch missed the fact that the corners kind of poke out toward the faux wheels.  So the assembly of 4 sections is actually a little wider than an individual section.

I put a black circle in the corner to represent the ball, but it isn't accurate.

Also they go up under the skirt a bit, top might need shaved if there isn't room.

The Lego Greeble Position WRT the Skirt on the Sentry Droid

I expect the edges to be about 1cm inside of the outside edge (or about a lego brick width) on the bottom. I'm not quite sure, but there's a lip.