Le top usa has a musical project to make a diy banjo using a paper plate, rubber bands, and a paint stick.
Good for ages 6 and up.
Get the instructions here:
http://letopusa.wordpress.com/2012/02/03/paper-plate-banjo-craft-for-fun/
Martha Stewart also has a banjo tutorial involving a shoe box, rubber bands, some wood pencils and a paper towel roll for the base.
Get the instructions here:
http://www.marthastewart.com/265694/making-music