  J-PET is a photon detector built of plastic scintillators, which already has
been commissioned for CPT studies in the decays of positronium. In the first
experiment, J-PET has achieved a sensitivity to CPT violation at a level of
10^{-4}, and now it aims to reach a level of 10^{-5}. This will be done by
enhancing the three-photon registration efficiency for ortho-positronium decays
using a new layer of densely packed plastic scintillators termed Modular J-PET.
We present the simulation studies performed for different experimental
detection setups to be used for the next CPT test with the Modular J-PET
detector.
