Review on 📦 NavePoint 1U 19-Inch Fixed 4-Post Rack Mount Server Shelf – Black, Adjustable Depth: 18-29 Inches by John Pound

Fits on a flat rack eg 19-22". In addition you should also start by fitting the front which means that the front of the shelf is pushed backwards. They made an odd design decision. Typically, the rack mounts solidly in the front; and have adjustments in the back. They're symmetrical, which means that with deeper racks (e.g. 25 inch or more) the solid part of the shelf is in the middle.