I guess a rail kit or we use navepoint adjustable mount server shelves 1u at work. I've used some of these damaged ones and cut them down to fit my 15' depth rack, the servers just slide into them but it can be a bit tricky to space them out correctly so the next server fits without clipping the one underneath or above

https://www.amazon.com/NavePoint-Adjustable-Mount-Server-Shelves/dp/B0060RUVBA/