Various apparatuses and methods relate to coolers or other containers having attachment features to which modular accessories may be attached. Various apparatuses and methods relate to wheel and modular accessory attachment systems for attaching to a cooler or other container. Various apparatuses and methods relate to rails and channels for sliding attachment couplers. Various apparatuses and methods relate to keyed male-female attachment couplers. Various apparatuses and methods relate to a chassis configured to adjust in size for adapting the chassis to fit a variety of container sizes. Various apparatuses and methods relate to a coupler assembly for mechanically securing a container to a container undercarriage system.
