Passive Hardware Components
Several members of the passive hardware component group will take the vast majority of your design focus: cables and power distribution units. First of all, ensuring that the cluster's rack PDUs can provide power to all of the active devices is important. Understanding the power requirements of the racked components and then matching those requirements with the proper power distribution is a key design step. It's also fundamental to ensure that the computer room has the correct type of power connections (to match the racks), so there are no embarrassing delays once the racks are moved into place.
Along with all the nuts and bolts, mounting rails, and power trips, there are usually a lot of cables to worry about. Designing a proper cable-documentation system, or borrowing one that is known to work, will make debugging and maintaining the cluster much easier. In a nutshell: Apply unique serial numbers to each cable, labeling both ends with the serial number, and then track the end connection points in a spreadsheet. You don't want a system that requires relabeling the cable if the connection is changed.
Also remember that some cables might be installed under a computer room floor, between racks. I refer to the two type of cables as intra-rack and inter-rack cables. Intra-rack cables run between two or more devices in the same rack. Inter-rack cables run between two or more devices in separate racks. Intra-rack cables can be run when the rack is assembled, but inter-rack cables (at least one end) must wait until the racks are placed in their final positions.