Lowe's also had this larger mortar box that would also make a very functional SIP. A tote box would be cheaper, but I like the black plastic and the shape.
If you don't like the plastic, make a simple wood box frame and it will become a raised bed SIP. Use any recycled lumber of your choice. It won't be in contact with moist soil.
Several of these would make a cool raised bed garden on concrete or blacktop.
So many SIPs to make, so little time.
36"x24"x8" - 23 gallon capacity - 3.2 cu. ft. capacity - $12.48
These are the dimensions of the smaller one (Fire Escape Salad Bar).
26"x20"x6" - 9 gallon capacity - 1.3 cu. ft. capacity - $5.24