The core of the probe is three 4xAA battery holders glued together. The top one have no battery connectors left but instead are used to attach the Feather and sensor. The two bottom battery holders are wired in parallel to the battery-port of the Feather.
At the bottom there is a bunch of steel balls acting as a weight to make sure the probe is in the right orientation in the water. The steel balls is encased in wax to make a form that fits in the bottom of the tube and creates a flat surface for the core to rest against.
At the top there is a 3D-printed collar to make a snug fit against the top of the tube, that collar are hot-glued to the tube and on top of that a thin plastic film that are sealed of with silicone.
As an antenna this version uses a simple wire cut to length for 433MHz, the wire is in held in a coil around the top most bit of the core. The next version will have a proper antenna on the outside of the tube, somehow.