If a fake leg cast is needed for a Halloween costume or a school play, use paper mache. Paper mache replicates the look of a plaster cast but is lighter, easy to apply, and uses inexpensive materials. Thin paper mache is easy to cut with strong scissors, and the surface can be sanded or painted as you see fit.
Obtain a mannequin leg from a mannequin supply store or online (see resources).
Completely wrap the mannequin leg with cling film. Apply tape along the edges of the wrap as necessary to hold it in place.
Make the paper mache paste by measuring 2 cups of white flour into a mixing bowl. Gradually add water to the flour while beating with an electric mixer. Stop when the paste reaches the consistency of pancake batter.
Tear the paper bags into 2-in. wide strips. Dip the strips into the flour paste, remove and wipe off the excess paste.
Apply the strips to the mannequin leg, overlapping the strips as you go, until you have covered the area desired for the cast. Allow the first layer of strips to dry and apply more layers as necessary until the cast is the thickness you want.
Cut the cast down both sides of the mannequin leg using heavy scissors. Paint the outside of the cast pieces white.
Place the paper-mache cast pieces on your leg. Cut strips of white fabric twice as wide as needed to fit over the seams. Glue the fabric in place and allow the glue to dry. The cast can easily be removed by cutting it along one of the seams, and reused by gluing a fresh strip of fabric over the open seam.