Bruno, its mostly simple material.
The base is an internal house door (light, but strong and cheap)
The side walls and pits at the back were made from timber off cuts I found at my dads place; cut to size, then glued and screwed to the base.
The fence posts were made from kebab sticks, cut to length, glued into holes drilled into the base and painted.
The fence was 2 metre length of flywire, cut to size
The base, walls and buildings roadways were painted with Dulux 'suede' texture paint using small sample pots ($10 each).
Finally, I applied fake grass, using material bought from a hobby store (also $10 each):
To apply the grass, I applied a water based glue that was diluted 50% with water so it could be brushed on. The grass was then sprinkled over the glue, but after trial and error I found I had to use a sieve to get it even and avoid clumping. Nevertheless the particular brand of grass I obtained was still a bit coarse and if I did this again I'd look for a finer material.
Once the glue had dried (it dries clear), the base was turned upside down to remove the surplus material. Obviously you need to ensure the glue has fully dried before you place any models on it.
“Racing is life. Anything before or after is just waiting.” - Steve McQueen