perhaps new york fries solved the plastic lid problem? their fountain drinks come in a fold-top paper-ish cup. might not be tight enough seal for hot liquids.
but if you made a sticky strip under the lip of the 4th flap (make it water activated so a quick wipe with a wet rags makes it stick), or maybe just use a large sticker to seal. probs as tight as a platic lid, and structurally better than a plastic lid (which amamzes me that given the millions of daily car window cdrive-thu and carried by tray and etc there aren’t many stories of bad burns. )
here’s a few solutions for when you want the “lid” to be solid, or at least as solid as we get with the current platic lids.
