Housing or fixture or enclosure provides physical safety to reef-pi electronics and is a must-have for any permanent installation. Housing or enclosure secures the electronics from outside environment, as well as provides a mounting place for the different probes, equipment and display panels. There are many ways one can build housing for reef-pi. Couple of things to consider:
Depending on what all features you want to use in reef-pi, ensure the housing can accommodate all parts. It is a good idea to leave extra space for any additional circuit or parts you may need in future. A typical lighting or ATO only controller can be built using 4x6 inches sized housing. While controller involving electrical receptacles may need much bigger space and the actual size may depend on the number of electrical sockets you want to control
The controller housing will be used to mount various connectors, and the wall thickness of the housing will be a limiting factor. Most audio jacks and barrel connectors has 5mm to 6mm wide threaded base, so make sure the housing has wall thinner than 5 to 6mm. Otherwise it will be hard to mount the female panel mount jacks.
It is popular to use plastic (ABS) project box or any other ready-made enclosures for the build. While they are sufficient for most builds, consider fabrication requirements while choosing one. For mounting jacks and sockets you may need to drill holes or cut shapes in the housing. For electrical sockets, it is important to use a housing with electrically safe materials. Personally, I use a lot of wooden housing which is why most guides might have images for wooden housing, but this is only because I am comfortable with woodworking. I do not recommend going this route unless you are familiar with wood working.
The recommended Bill of Materials contains an example enclosure. We highly recommend you to think through your controller features and electronics before deciding on an enclosure. The Sparkfun guide provides some helpful tips on how to do this.