Here’s a post on Instructables (by way of Craft:) on how to fuse plastic grocery/shopping bags together to create a cool messenger bag.
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You can use any kind of bag for the top layer to create a fun look for your bag. Here they’ve used a bag from a liquor store as the top layer/outside pattern.
This is a really neat idea, but I must reinforce their warning: ***do not heat these bags to the point that they melt because you do not want to inhale the fumes! Please do this in a well-ventilated area (outside?)!