Or more accurately, cobbles. I've decided to base my rioters on cobbled streets, as per the test element below.
The core of the bases are 60mm square galvanized steel sheet which I bought them off Renaissance Ink some years ago. I've cut and fixed the ubiquitous Magnetic sheet on the back, and primed the top with Halford's Grey to disguise the shine on the edges (see below, I've turned one over to show the rear, you can see the shiny steel). I joined a couple of the bases together to take the cart, which was too big for a single base.
The Roman cobbles (below) are actually from Antenocitii and are printed into a soft-ish styrofoam, and supplied pre-painted; a great product. This is what I'm going to use for my Roman streets, so the rioters should blend right in. Below are the 11 bases I'll need for the units.
As per the test piece at the top, I will need to cut a hole in the foam for each mini's base, and stick them to the metal, then lay cobbles around the feet to disguise the base. This worked more or less OK with the test base in the top figure, not as hard as one might think, albeit fiddly; but with 46 minis plus furniture, I clearly have a few days basing duties ahead of me!