Banana1008a via eBay offers this Thermal Waterproof Lunch Bag in four colors (Sky Blue pictured) for $1.69 with free shipping. (Yellow is available for a penny more.) That's a slight drop from last month's mention of similar bags and the lowest total price we could find by $2 (outside of other eBay sellers). It features aluminum insulation to keep food hot or cold.
Note: Allow up to three weeks for delivery.