I recall reading about making sacks from modelling clay, as Andrew suggested. I don't know where the article is, but it was pretty simple. Take a lump of clay, roll it out into a "snake" about the same diameter as the size of the bags you want, then clip off individual bags using diagonal cutters. This gives the effect of the closed ends. If you want a stack of bags laying on their sides, flatten the rounded part out a bit with your fingers. If you need bags standing upright, flatten out one of the crimped ends, either manually or with an X-acto knife. If I recall correctly, the article mentionned baking the bags to harden the clay, then paint them to suit.
Wayne