Many times in life you will need to mount a device in a rack without the assistance of a friend.
Sometimes you will need to leave a 1RU space in between devices for thermal reasons.
Sometimes supporting the device while you install the rack screws can be a great challenge. I’ve been known to use my teeth, knees, toe, etc…
I’ve also used audio gear as a spacer before which works if you have a spare 1RU, 2RU, 3RU audio device handy but you run the risk of trapping the device in the rack.
For those of you who may not know what a “rack unit” is, here is a wiki article that explains it.
So 1RU = 1.75″ (inches) tall and 19″ +/- wide
For reference, this is one of my Glyph hard drives measured with a digital caliper. 1 & 23/64 inches. Obviously if a RU is 1.75 inches the gear itself cannot also be 1.75″ or the tolerances for both the rack and the gear would need to be absolutely perfect. So manufacturers undersize their gear just a bit to make sure it fits. Rack manufacturers also build a bit larger than 19″ wide. I’ve had racks that were almost 19.25 inches. In this case the gear I own wouldn’t line up with the rack holes. Most of the racks I’ve measured are 19 1/16″ or 19 1/8″ wide. Plenty to fit a 19″ wide device.
What might make a good spacer that is inexpensive, readily available and easy to manange? A modern wooden 2″x4″ is roughly 1 & 1/2″ x 3 & 3/8″.
What this means is that (2) 2″x4″ scraps make great 1RU spacers to support gear while you install the screws. If you make the 2″x4″ scraps long enough and have them stick out a bit, you can use them as a lever to adjust the height of the device you’re mounting.
I now keep a pair of these wooden spacers in my kit for when I need to shuffle gear around in a rack. They are a bit on the short side so I might put some felt on both sides to make them closer to the 1.75 inches. This will also keep the gear above and below from being scratched by the wood.