  We recently proposed a high-granularity calorimeter insert for the
Electron-Ion Collider (EIC) that is based on plastic scintillator tiles readout
with silicon photomultipliers. In this work, we concretize its design by
characterizing its building blocks with measurements of light yield, optical
crosstalk, and timing resolutions using cosmic-rays, an LED, and a beta source.
We also compared two approaches for the optical isolation of cells:
``megatiles'' with grooved boundaries between cells, and a 3D-printed plastic
frame hosting individual cells. We found that the latter suppresses optical
crosstalk to negligible levels while providing an easier assembly method.
Overall, these performance studies can help inform calorimeter design and
realistic simulations of 5D showers (time, energy, position) for the EIC and
other experiments.
