6 years ago I bought a 12" (Width x 20" (Depth)) bottom drawer made of stamped steel sides and a thin plywood base. which sits on the sides of a metal steel box that has been bent 90 degrees to provide a 1/2" wide steel "edge." Over the years this box has fallen despite only weighing a few ounces , off the rails several times, which I can't remember as the steel was too thin so the sliders gradually slid off the sides of the crate, allowing it to fall off the edge of one of the side pieces. No ball bearings, just metal. With these With new keyboard drawer extensions in hand I drilled 4 small holes in the steel edge and plywood that sit on each edge and used them to bolt them to the steel edges on the bottom pieces of each side of my old box.I then screwed the extensions to the underside of my laminated table. The steel is 2-3 times thicker than that used in the original drawer runners, while ball bearings and a "stop" feature ensure smooth sliding. Now I have d he same box as before, but it's finally *tough*.