It’s easy to forget, when loading gear in a rack, that some pieces generate a substantial amount of heat — and heat is a primary foe of electronics. At the least, since heat rises, ideally, you would place an item that generates a lot of heat at the top of the rack.
But a better solution is to leave a blank rack space between heat-generating equipment in the rack, then use a vented blank rack panel to fill in the front. This allows for air flow and for the heat to escape without endangering other components. You can even get hinged rack vent panels that swing open to allow access to gear or connections — perfect for dressing up the rear of rack and hiding cabling.
For a really hot rack, you can install rackmounted fans to help circulate air and keep the heat under control.