Most require quite a bit of effort and know-how to make well, but a sling pack doesn't require too much sewing skill. Instructables user linrodann put together a simple set of instructions for making your own on the cheap.
Linrodann calls it a hobo bag, but it looks just like any standard sling pack. The project only requires your fabric of choice, standard sewing materials, and the included template. The downside is that you have to tie the bag to secure it to your body, but the addition of a velcro strap, a button or a clasp can easily solve that problem.
Check out linrodann's full post on Instructables for the detailed how-to.
Sew a Hobo-Bag [Instructables]