Well, I worked out a nice method using castor sugar and white glue, and tested it yesterday on a figure base.
Today I went shopping!
I got three foam board sheets - greyish ones, rather than white ones, as they were much cheaper.
I started by squeezing out lines up/down and sideways across the board, then with a very wet brush, wiped the glue all over the surface.
Then I sifted sugar from the bowl, in a diagonal pattern, until the whole board was covered with sugar.
Then I dry-brushed across the surface of the board, spreading the sugar into the glue, and also it made a sort of rough patches and interesting textures with the sugar/glue.
I waited a little bit, then I covered the board again, much thicker, piling sugar on in places.
Tomorrow I will brush off all the piles of loose sugar (making sure to collect it so I can use it on the next board) and then thinly brush glue all over the surface of the board again to seal the surface and give it a good icy shine.
The idea is that I start the first board today, then tomorrow it is ready to finish, and I start the second board, then the next day it is ready to finish, and I start the third board, and then the fourth day, it is ready to finish, and I end up with three boards than can cover my table.
I'll be adding small rocks, gravel and some small patches of dirt/grass in some areas of the boards as well.
It will be pretty flat - no undulating hills or anything, but it should have a nice texture to it. I can then sit scenery objects on top of it. I actually wanted a big 4x6 foot gaming table, but I don't have room in my place for a table that big to be set up - and to be able to walk around it to play.
This table will be around 75cm x 150cm, or 30" x 60". (2.5 foot x 5 foot) as that is the shape of three foamboard sheets side by side.