It's a small case/box for e.g. a DHT22 or DHT11 sensor. In a seperated compartment I placed a ESP01/DHT board with a small psu. There is a small area where you can tie wrap the incoming power cable.

You get one of those ESP01(S) DHT boards (see pic example), remove the DHT sensor* from the board and add some wires to it*, to place the DHT sensor in the seperate compartment. 
*Requires some soldering.

The DHT board can clip into place. Please make sure to measure first and calibrate printer (or just print the bottom part of the case and test fit beforehand and make adjustments accordingly).

Please place some insulation in the highlighted area. There is a small slot down to the bottom to be able to get a wire in before adding the insulation. With the insulation in between the two compartments the heat from the esp/psu do not influence the DHT sensor. There is a hole at the other side with a small ‘seat’ for a tiewrap to fixate the mains cable for the psu.

I used some heatset inserts and m2 screws.

Now I can just plug the thing into mains somewhere and get the temperture and humidy reading from there right into home assistant.