If you are very tight on space, Inteset Technologies sells a computer case that is rackmount compatible where you're able to fit two complete systems within a 1U slot.
The Inteset TX482-1U is a thin mini-ITX computer case that can be shelf or rack mounted. However, for rack-mounting you also need to purchase a separate face-plate that makes it rackmount compatible. The good thing though about this face-plate is that two of these cases can be mounted side-by-side to have two systems running in a 1U slot.
Recently I ended up picking two of the TX482-1U cases plus the Inteset faceplate to be able to squeeze in two systems within 1U. One of the systems was a NVIDIA Jetson TX1 development board where as the other mini-ITX system was the Habey MITX-6771 that features a quad-core Celeron J1900 SoC.
The Inteset TX482-1U is quite a simple case, but it gets the job done. Internally it can accommodate a single 2.5-inch drive and a mini-ITX motherboard. Besides being rack-mount friendly, this can also be VESA mounted, car mounted, etc. On the front of the unit are two USB 2.0 ports, audio jack, and the power switch and LED indicators.
At the rear are some plugged holes as well for accommodating any extra antennas. There is also a secondary hole for a PSU connection if not found on the I/O panel for the motherboard. The case requires a half-height back plate for the I/O panel. Also at the rear is a 40mm exhaust fan.