Before I ran to the store to get some fabric to sew together for a DIY pouch, I decided to do a super-simple hack that has ended up being my long-term solution. This has lasted me over 250 miles so far without a problem.
All you need beyond the Nike+ kit is a knife and a safety pin. I don't really know how much a safety pin costs, but I'd guess 5 cents.
If you're not familiar with the Nike+ system, you can check it out here: