A few hours last Saturday and I had a pattern worked out. Another hour or so this Saturday and we are stocked up with four larger sandwich bags and four smaller snack bags.
The outside is cotton scraps from the closet stash. The inside is pieces of an old tablecloth - the kind that is vinyl on one side and flannel on the other. We use a table cloth like that on our dining room table at all times. A single tablecloth lasts almost a year (if you include using clear mailing tape to patch holes for a while). Once it becomes too ratty for the dining room table, it then gets demoted to a craft/painting dropcloth. We are on our second tablecloth now and I KNEW I saved the first one for a reason. :) Just made sure there weren't any paint drips on the pieces I cut out for lining and we're good to go! I also had spare velcro in the dresser so that means each of these bags cost me exactly zero dollars to make. Even better!
Velcro closure - hoping it won't get too nasty with crumbs and whatnot. I plan to keep them clean by either just brushing out the crumbs, giving them a swish around in the dishwater and let them air dry, or they can be tossed in the regular laundry.
Leah already claimed the pink crosshatch one as her 'lunch sandwich bag' since it'll match her pink lunchbox. :) I did make one green on purpose so as not to make Keith eat out of a pink or purple bag at work. (He usually uses tupperware anyway, so not a huge risk...)