I think I'm actually sitting on an idea, albeit probably better suited to indoor use. It is a kind of polycarbonate sheet that goes under the office chair to protect the laminate flooring from the casters. One side is rough and the other smooth, in my case the rough side is up but for modelling I would paint the rough surface then have the smooth surface uppermost (instant 3D effect). I got mine from the carpet department of a furniture suppliers - and here's the thing, I had to sort through a box to find an undamaged sheet but for modelling that isn't a worry so a reduced price could probably be sorted.