CsI(Tl) is a commonly used scintillator with good energy resolution because it has a very large light yield, around 54 photons per keV. It is slightly hygroscopic but not enough to cause serious concern for testing. I have 10mm x 10mm x 30mm CsI(Tl) crystals that I wrapped in Teflon and used clear 3M VHB tape for coupling them to the SiPMs. A black 3D printed housing for the crystal acts both as mechanical protection but also as a light shield.

Directly underneath the SiPM on the PCB there is a high accuracy SPI temperature sensor (TMP126). The SiPM gain depends on temperature and can be compensated for by adjusting the bias voltage. However, this functionality is not implemented in firmware yet.