I am building some large cabinets for shop storage. sides are 3/4" plywood, shelves are 1/2" plywood framed with 1.5x1.5" wood. Even screwed and glued together is can rack. So the back (this would be akin to the rear shelf bracing here) is a sheet of 1/4" plywood screwed and glued to the sides and each shelf as well as the top and bottom. It will never rack.

If you would add a sheet metal cover atop that bracing to both seal off the entire opening but you welded it to all the sides and braces you'd pretty much never have flex there. Over kill? Maybe, but I do';t have a supercomputer to run numbers. I know some sanctioning bodies require that to be sealed off if you are running a fuel cell so it may as well pull double duty.

They say there are no such thing as a stupid question.
They say there is always the exception that proves the rule.
Don't be the exception.