Another option to the string and nail is a piece of wood about 1 inch by 1/4 or less and 40-60 inches long. Basically a the wood should be like the old wooden yard sticks. Drill a hole 1 inch from one end the size of your nail. Then at the other end drill holes the size of a pencil. Measure from the nail hole to the smallest radius curve and drill the first hole, say 31 inches. Then drill other holes at the radius for other curves you want to use. You can always add more holes later as needed. You could put them at 1 inch intervals if you want.
The wood stick is a little easier to work with than the string. A friend in HO taught me this trick and I tried it. I have my "radius" stick hanging on the wall in the garage.
