YAKIMA, EXO TopShelf, Second Level Base Rack The YAKIMA EXO TopShelf is a fantastic addition to your hitch storage setup. With this clever add-on, you can double your storage space on your hitch. Simply attach the EXO TopShelf to the EXO SwingBase (sold separately), and you'll have a second tier of storage for all your gear. One of the standout features of the EXO system is its adaptability. The EXO Cleat system and SpeedKnobs allow you to easily swap out EXO components (sold separately) as needed. Whether you're biking, skiing, or camping, you can use the same base rack system for all your adventures. Convenience is key, and the EXO TopShelf delivers. This hitch-mounted system makes accessing your gear a breeze. It even rotates a full 180 degrees relative to the EXO SwingBase, ensuring easy access to the lower level of storage. Installation is a breeze, and the EXO TopShelf locks securely in place. It comes with a HitchLock to secure it to the EXO SwingBase, providing peace of mind when leaving your gear unattended. Additionally, two extra SKS lock cores are included to secure your gear and EXO components. If you're setting up a base camp, the EXO TopShelf is a must-have. It includes a convenient jack stand and pairs perfectly with the EXO BackDeck (sold separately) to create a multipurpose table for all your camping needs. Now, let's address some frequently asked questions: Can I mount my Thule cargo box? I am not sure if Yakima's attachment system is proprietary or not. I suspect that a Thule cargo box wouldn't have the same receiver hardware (ie: the tool-free EXO Cleat system and locking SpeedKnobs) necessary to attach, secure and lock into the Yakima TopShelf. I can understand wanting to use an existing Thule cargo box but I don't think it would secure properly without the EXO Cleat system hardware. In all likelihood, you will have to purchase an EXO GearLocker cargo box if you want to bring along extra gear (which is an added expense as they are not cheap ... $400+). What is the OD of the square tubing? 2" or 2.5"? The vertical tubing has an outer diameter of 2.25", while the horizontal tubing is 2". The arms that hold the cargo have a diameter of 2.25".