One half-cup of corn starch (and maybe a little
more)
one half-cup boiling water
double boiler
stove (hot plate)
flat pan or cookie sheet
Procedure
Mix the sand and cornstarch thoroughly in the top of the
double boiler.
Pour in the boling water and mix well (but carefully -
its hot!)
cook in the double boiler briefly until the mixture has
thickened. Should it be too thick, add a little boiling
water.
Wait a moment for cooling and the clay is ready for
modeling.
Take out the clay and flatten it gently with your hands
(imagine you are making a thick pizza) - you can even use
a rolling pin.
Using some tool the size of your playing stones, copy the
board grid and the groove around the board by pressing
into the clay.
When the board is finished put it on a flat pan or cookie
sheet in a 275 F (132.2 C) oven until dry. It can be
dried without the oven but it will take longer.