Measuring electron temperature is an important method to understand the stability and coherence of a quantum circuit, since this variable describes how `quiet' the electronic environment is. In this thesis, the construction, calibration and operation of a quantum dot electron thermometer is demonstrated in two different cryostats. Compared to previous implementations of a quantum dot thermometer, the work presented here is unique in that it only requires a single gate connection to calibrate and operate, which simplifies the application of the device substantially. For the thermometer calibration, a physical model of the quantum-dot reservoir system was developed, which reveals information usually obtained from a stability diagram. Electron thermometry was successfully performed with the calibrated thermometer in a 1.0 K to 3.0 K range. With the fastest mode of operation the quantum dot thermometer was shown to have a sensitivity of 3.7±0.3 mK/√Hz at 1.3 K. This device provides a new versatile, sensitive and effective tool for monitoring electron temperature in nanoelectronic devices at cryogenic temperatures. Also in this thesis, several plastic solid-void structures were demonstrated to offer excellent thermal and structural properties at sub-Kelvin temperatures. Good low temperature insulators are extremely useful for support cryogenic components and sample environments without leaking unwanted heat. A structure fabricated from commercially available ABS LEGO elements was shown to be effective at thermally insulating two bodies at sub-Kelvin temperatures, with a thermal conductivity of κ = (8.7±0.3)×10-5 T1.75±0.02 Wm-1K-1. Similar scale 3D printed ABS and PLA gyroid structures were shown to also be effective as low-temperature insulators, having a thermal conductivity of κ = (3.07±0.05)×10-5T1.72±0.02 Wm-1K-1 and κ = 4.45±0.05)×10-5T1.64±0.02 Wm-1K-1, respectively. These samples demonstrate how low temperature insulation can be improved with readily available, fully customisable and affordable components.
