What to use as the background for a self-made scratchboard (or scrapeboard)?
What you are looking for, the base for a scratchboard, is Clayboard. It is available in art stores and online... search for art panels.  

You can make clayboard a number of ways:


The Jimmy Arborough site has a recipe for rabbit skin glue based
ground. Rabbit skin glue is tricky in that it requires hot water, cooling time, and is made out of rabbits. He uses about a 1:1 ratio of water to clay in his recipe. 
The white glue and plaster method is popular and easy. Use plaster of paris not drywall compound to get a brighter white. 
You could also mix kaolin clay or plaster of paris with regular acrylic ground and apply that.  Here start with 1/3:1 ratio of clay to base, adding more clay to get the absorbency/feel you desire.


With any of these methods you can whiten the base by adding Titanium White. 

Once you have a ground, apply it in multiple layers to masonite or heavy paper board in layers, letting each layer dry between applications. You will need a number of layers (5-10) to get the right thickness. If the board warps, try painting the back with an X of the same ground to pull it back into shape. When the board has dried thoroughly, lightly scrape the surface with a knife to remove any blobs or burrs, then sand smooth with a fine grit sandpaper.  

To finish the scratchboard, take a dry and clean clayboard, brush or spray with India Ink, again layering until you have the tone you want. The result should be a smooth hard surface, ready for carving into.