Whoops... forgot that crucial bit of info. C4D r9 has Chlotilde which could possibly do this, and it'd build the whole bag at the same time. Check out the tutorials here (in the poser or cinema 4d section, can't remember which) for making clothes with the Chlotilde plugin, and I'm sure you could turn that into your plastic bag if you have C4d. I'm not sure which other programs have a similar function specifically. Sorry about the P4/p5 mixup.