More ideas from Bori
No instructions just this picture of a zippered picnic / lunch bag that turns into a mat.  Could study the pix to see how they did it.  Neat looking bag though

Rita Floyd-Vester is raising funds for FlatBox-LunchBox - Folds Out to a Placemat on Kickstarter! FlatBox-LunchBox is a washable, heat resistant lunchbag that unzips and converts into a placemat providing clean surface for its users.