Open racks are pretty easily cooled with various options for cooling and we'll present some recommendations but there's not going to be too many right or wrong ways, just really preference based on the application and available space. Typically racks are made to mount fans in the tops and sometimes in the bottom so the first and obvious option is to simply utilize those spots for cooling although, in open racks you may want to concentrate more directly on the heat producing components which may actually be in the bottom of the rack. CCI offers fan kits for mounting in most any rack top or bottom.
Products for Front Mounting. This section makes the assumption have room to mount products in the front of the rack between other components. The #1 recommended product for rack cooling is the Universal Cooler (CP-UC) as it only takes up 1U, can be mounted front of back, it can be configured to blow air in or out of the rack, upwards or downwards. It really is universal and a nice amount of airflow which can cool most any component you can put in a rack. Other commonly recommended products are the 2U Universal Cooler (CP-UC-2) and the Component Units (CP-CU) and there's various versions of the Component Units, probably the most recommended is the 4-Fan Unit with Cover as it moves air directly into, or out of the rack, not just blows air upward or downward.
Products for Mounting Elsewhere. This section is for when really do not have space to mount cooling products on the front of the rack and do note that depending on the type of rack, usually products can also be mounted on the back rails as well. The best units we have space challenges open racks are the Side Coolers (RK-SC) which come in 2 and 4 fan versions. These simply mount to the side of the rack for easy access and very direct cooling of components.
"In-Between" Concept. OK, this section is a stretch but if don't have extra space for separate cooling products, consider the Cool Shelves that simply take the place of standard rack shelves but have cooling built into the shelves. The Cool Shelves (RK-CS) come in 2, 3, and 4U configurations and are, well, very cool!