  The search for Galactic PeVatrons - astrophysical accelerators of cosmic rays
to PeV energies - has entered a new phase in recent years with the discovery of
the first Ultra-High-Energy (UHE, $E>100$ TeV) gamma-ray sources by the HAWC
and LHAASO experiments. Establishing whether the emission is leptonic or
hadronic in nature, however, requires multiwavelength data and modelling
studies. Among the currently known UHE sources, LHAASO J2108+5157 is an
enigmatic source without clear association to a plausible accelerator, yet
spatially coincident with molecular clouds. We investigate the scenario of a
molecular cloud illuminated by cosmic rays accelerated in a nearby supernova
remnant (SNR) as an explanation for LHAASO J2108+5157. We aim to constrain the
required properties of the SNR as well as which of the clouds identified in the
vicinity is the most likely association. We use a model for cosmic ray
acceleration in SNRs, their transport through the interstellar medium and
subsequent interaction with molecular material, to predict the corresponding
gamma-ray emission. The parameter space of SNR properties is explored to find
the most plausible parameter combination that can account for the gamma-ray
spectrum of LHAASO J2108+5157. In the case that a SNR is illuminating the
cloud, we find that it must be young ($<10$ kyr) and located within $40-60$ pc
of the cloud. A SN scenario with a low Sedov time is preferred, with a maximum
proton energy of 3 PeV assumed. No SNRs matching these properties are currently
known, although an as yet undetected SNR remains feasible. The galactic CR sea
is insufficient to solely account for the observed flux, such that a PeVatron
accelerator must be present in the vicinity.
