  Interstellar clouds can act as target material for hadronic cosmic rays;
gamma rays subsequently produced through inelastic proton-proton collisions and
spatially associated with such clouds can provide a key indicator of efficient
particle acceleration. However, even in the case that particle acceleration
proceeds up to PeV energies, the system of accelerator and nearby target
material must fulfil a specific set of conditions in order to produce a
detectable gamma-ray flux. In this study, we rigorously characterise the
necessary properties of both cloud and accelerator. By using available
Supernova Remnant (SNR) and interstellar cloud catalogues, we produce a ranked
shortlist of the most promising target systems, those for which a detectable
gamma-ray flux is predicted, in the case that particles are accelerated to PeV
energies in a nearby SNR. We discuss detection prospects for future facilities
including CTA, LHAASO and SWGO; and compare our predictions with known
gamma-ray sources. The four interstellar clouds with the brightest predicted
fluxes >100 TeV identified by this model are located at (l,b) = (333.46,-0.31),
(16.97,0.53), (110.43,1.89) and (336.73,-0.98). These clouds are consistently
bright under a range of model scenarios, including variation in the diffusion
coefficient and particle spectrum. On average, a detectable gamma-ray flux is
more likely for more massive clouds; systems with lower separation distance
between the SNR and cloud; and for slightly older SNRs.
