This is a case that will hold an Arduino Nano, a 9V battery, switch, run of 3mm LEDs and a probe tip. I use it for a CF/EC meter based on http://www.geekofarm.com/?p=109 \
There are 2 versions: The normal and the long. The long one has the probe and the component box all in one long bit; this might be a bit much for smaller 3D printers, hence the short version.
The LEDs are held in by friction fit on a good day, otherwise add hot glue.
The case is held together with 4x M3x40mm screws, 8 washers and 4 nuts appropriately distributed in the holes provided.
The size of the battery compartment, wiring cavity, probe length, electronics compartment and number of LEDs is configurable in the SCAD file - and there's a spot for an LED that can be drilled out if you find you need an extra one!