Im in mid progress making foam hills for the N gauge, I find its very quick, pretty cheap and simple to do.
In this case I have used B&Qs own brand 750ml aerosol for £7.98
I didn't take pictures of the spraying, the only important thing is to invert the can when spraying.
This is a picture taken about 45mins after spraying.
At this point the foam has nearly set but has a lot of air inside the still tacky inside. By poking a few holes in the skin a lot of the air can be squashed out to give a better look.
And here is a video of the foam being moulded.
Its not critical to get an exact contour as I will be covering it all with mortar or plaster.