MUKCHAP 12 x 12 x 3/8 Inch Aluminum Sheet Plate, 6061 Aluminum Sheet with Protective Film, 6061-T651 Aluminum Tooling Flat Plate

This is the 12x12 3/8 you want. It is well cut and perfectly surfaced. Oddly, it also costs substantially less than the other offerings on Amazon which are all a far cry from this quality (I have ordered multiple). If you need this size, order this one.

This item is mis-advertised as 3/8 (.375) inch thick. The actual thickness is .404 inches. Not close.

This 3/8 x 12x12 Aluminum stock is not only as described , its perfect, Just what I need. Thanks

You just can’t find a plate like this at a local hardware store. On Amazon, no problem.