In order to prevent access to the amplifier's controls and connectors, there are smaller, simpler and cheaper options than building a box. The simplest, from a customer's perspective, would be to cut a piece of plywood about the size of the back of the cabinet and drill some holes in it. Cut a notch near the middle of the board on each side then drill 4 holes that line up with the cabinet feet. Remove the foot bolts and, leaving the rubber feet in place, add the board using longer M8 bolts through the feet.

The notches will allow the cables to exit at the bottom of the board and the spacing provided by the feet will allow the cables to access the back of the cabinet and allow enough air flow for cooling. (Most of the cooling happens inside the cabinet thanks to the air-flow through the ports.)

Example below is for the ZV28: