I was wondering if I could use round tubes in place of the square tubes. If I mounted it horizontally I could pivot the load on the tube sideways. Imagine a vise's base plate welded to the side of the inner tube. When I want to store it I can rotate it on the tube and slide it underneath the table. This way a very heavy vise does not have to be dead lifted vertically and placed in a tube; it just has to be rotated upward.
If designed correctly, the base plate can slide onto the table top in the upright position. A large set screw on the side of the outer tube would secure the load in place.
Similar plates could be used for mounting a grinder and even a chop saw. All positioned around the table so their tubes do not interfere with one another.
Can I buy steel tube that has a slip fit into one another?