  We present an assessment of the accuracy of common operations performed in
$21$-cm spectral line stacking experiments. To this end, we generate mock
interferometric data surveying the 21-cm emission at frequency $1310<\nu<1420$
MHz ($0.005<z<0.084$) and covering an area $\sim 6$ deg$^2$ of the sky,
mimicking the observational characteristics of real MeerKAT observations. We
find that the primary beam correction accounts for just few per cent ($\sim8\%$
at 0 primary beam power, $\sim 3\%$ at 0.6 primary beam power) deviations from
the true $M_{\rm HI}$ signal, and that weighting schemes based on noise
properties provide unbiased results. On the contrary, weighting schemes based
on distance can account for significant systematic mass differences when
applied to a flux-limited sample ($\Delta M_{\rm HI}\sim 40-50\%$ in the
studied case). We find no significant difference in the final $\braket{M_{\rm
HI}}$ obtained when spectroscopic redshift uncertainties are accounted for in
the stacking procedure ($ \Delta z\sim 0.00035$, i.e. $\Delta v \sim 100\,{\rm
km\, s}^{-1}$). We also present a novel technique to increase the effective
size of the galaxy sample by exploiting the geometric symmetries of galaxy
cubelets, potentially enhancing the SNR by a factor $\sim\sqrt{2}$ when
analyzing the final stacked spectrum (a factor 4 in a cubelet). This procedure
is found to be robustly unbiased, while efficiently increasing the SNR, as
expected. We argue that an appropriate framework employing detailed and
realistic simulations is required to exploit upcoming datasets from SKA
pathfinders in an accurate and reliable manner.
