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.
NavePoint 1U Black Vented Rack Tray Shelves - Universal Design, 8 Inches (210mm Deep), No Lip
11 Review
Optimize Your Server Efficiency with StarTech.com 1U Vented Server Rack Cabinet
11 Review
NavePoint 1U Vented Rack Tray - 14 Inches (350mm Deep), Black, Lipless Design
11 Review
StarTech Com Bracket Equipment Mounting WALLMOUNT6
10 Review