OT a bit, I "love" it when my friends wonder why do I use a piece of white plastic on my flash...then I can tell them "oh, this cost $200 and it multiplies the light output." Then I'll enjoy the stunned expression on them.
Basically any material that is white in colour and is hardy is suitable to be used as a bounce card. Coupled with a few rubber bands or Velcro (more elegant solution), and there you have it! If you are creative enough, you could even line one side with aluminium foil or golden foil to increase the ratio of light that is bounced back.