Devlyn, it can be done with plaster, foamcore board, or styrene. I prefer styrene becase it's easier to get straight cuts, takes any kind of paint, and doesn't have the mess of plaster. Styrene sheets that are .080 inches thick should be just right for code 83 track. Yu can get the styrene from most hobby shops or direct from Evergreen. Measure the center of the track and leave about 10mm of space for the flangeways. When you have piece that fits right, you can use that as a template for all your other center sections. The styrene sheet can just butt up against the rails on the outside of the track. You'll have to use .015 styrene shims under the main part of the plastic if you are paving a large area and want the spaces between tracks to be flat.
