  Free-space channels provide the possibility of establishing
continuous-variable quantum key distribution (CV-QKD) in global communication
networks. However, the fluctuating nature of transmissivity in these channels
introduces an extra noise which reduces the achievable secret key rate. We
consider two classical post-processing strategies, post-selection of
high-transmissivity data and data clusterization, to reduce the
fluctuation-induced noise of the channel. We undertake the first investigation
of such strategies utilising a composable security proof in a realistic
finite-size regime against both collective and individual attacks. We also
present an efficient parameter estimation approach to estimate the effective
Gaussian parameters over the post-selected data or the clustered data. Although
the composable finite-size effects become more significant with the
post-selection and clusterization both reducing the size of the data, our
results show that these strategies are still able to enhance the finite-size
key rate against both individual and collective attacks with a remarkable
improvement against collective attacks--even moving the protocol from an
insecure regime to a secure regime under certain conditions.
