Rackmount Rails in a short-depth rack can be somewhat tricky to figure out. Most server rails are not designed for short-depth racks, and thus won't work out of the box. This leaves equipment installers with either needing to modify existing rails or procuring custom rack rails for their equipment.
WLNet has had reasoanble success with using CyberPower's 4POSTRAIL kit, as it can fit in a rack as shallow as 18.5". However, in the case where rack equipment is short AND wide, it may be necessary to install the rails on the rack unit below the installed equipment and to invert the rails to support said equipment.