IIRC, the coolers are made of polypropylene and you will be hard pressed to find any glue that will adhere to it and form a strong bond. In any case, you will still want some kind of fitting to pass through the cooler walls completely. This will make for a sturdier connection and in the case of a double walled cooler, it will prevent liquids from entering the gap between the walls. What's the problem with using the common bulkhead fittings, either store bought or DIY. These work very well, are durable, easy to install and not expensive. I see no advantage in doing it some other way.