When I broke my arm the wire coat hanger that I modified so I could stick it between the cast and my skin without poking myself was a Godsend. But depending on the type of cast you have, it may not be so simple for a leg. Not medical advice, as it will degrade the dressing between the fibreglass and your skin. 

 Oh believe me I tried that but it's hard locked against my leg & theres nothing I can do :( oh i've never felt so crazy from an itch!