A couple of years back I had purchased some cork tiles 1' x 1' each and produced a small surface for 6mm scale models. I realised these work (perhaps better) for 15mm, so I purchased another two sets to add to the original two to have a 4' x 4' surface (which obviously can be reconfigured in any other way up to the maximum of 16 tiles).
I painted these using a grey spray tin, leaving the appearance irregular to obtain a patchy appearance. I sprayed some small lighter grey patches, then added a few specks of brown or tinier ones of yellow (they just looked good!) and some tiny clumps of cheerful bright green flock in cracks in the surface.
Here is half the surface (I realised I do not have a table large enough to set all these up on!).
A close up of a single tile:
Two tiny bits of scenery - more are required obviously!
And a tile with a ruler and the manufacturer's label - I purchased my first 2 packs of 4 from a local ironmonger and got the latter two from Amazon UK as I could not get more locally. Fortunately they were the same brand.