If you have trouble sourcing air-drying ‘clay’ you can make it fairly easily out of cornstarch and white glue plus a little oil.
The recipe I use is a cup of cornstarch to about 8 fluid ounces of white glue. Use a whisk (or whisk mixer) to blend them together until smooth with no lumps. Then add 2 fluid ounces (or 4 tablespoons if you want to use that measure) of cooking oil and stir in. I was also taught to add a couple tablespoons of lemon juice, but don’t know the actual effect on the chemistry with the glue. Microwave this about 30 seconds, take it out and whisk it to distribute heat, then 30 seconds more. Pour this on parchment (cooking) paper that has some cornstarch sprinkled on it an gently knead it around until it isn’t sticky to the touch.
When I used this for molding, I would let it air dry and then gently bake it a bit, then spray the front with a good varnish to seal and make it less likely to stick. Sprinkle a little talcum if it still does.
You may find it useful when rolling material that needs a consistent ‘cross section’ to use guide rollers either side of the formed ‘roller’ that run on rails of wood stock on either side cut and leveled to the right height.
I used this general technique to make ‘top down’ concrete roadbed profile complete with rolled-in ties and other detail for outdoor modeling of long mainline stretches; you can use both horizontal and vertical templates for curve transition spiraling with correct superelevation with a little care.
