  The progenitors of present-day galaxy clusters give important clues about the
evolution of the large scale structure, cosmic mass assembly, and galaxy
evolution. Simulations are a major tool for these studies since they are used
to interpret observations. In this work, we introduce a set of
"protocluster-lightcones", dubbed PCcones. They are mock galaxy catalogs
generated from the Millennium Simulation with the L-GALAXIES semi-analytic
model. These lightcones were constructed by placing a desired structure at the
redshift of interest in the centre of the cone. This approach allows to adopt a
set of observational constraints, such as magnitude limits and uncertainties in
magnitudes and photometric redshifts (photo-zs), to produce realistic
simulations of photometric surveys. We show that photo-zs obtained with PCcones
are more accurate than those obtained directly with the Millennium Simulation,
mostly due to the difference in how apparent magnitudes are computed. We apply
PCcones in the determination of the expected accuracy of protocluster detection
using photo-zs in the $z=1-3$ range in the wide-layer of HSC-SSP and the
10-year LSST forecast. With our technique, we expect to recover only $\sim38\%$
and $\sim 43\%$ of all massive galaxy cluster progenitors with more than 70\%
of purity for HSC-SSP and LSST, respectively. Indeed, the combination of
observational constraints and photo-z uncertainties affects the detection of
structures critically for both emulations, indicating the need of spectroscopic
redshifts to improve detection. We also compare our mocks of the Deep CFHTLS at
$z<1.5$ with observed cluster catalogs, as an extra validation of the
lightcones and methods.
