  This paper presents a comprehensive optimisation study to maximise the light
collection efficiency of scintillating cube elements used in the SoLid
detector. Very short baseline reactor experiments, like SoLid, look for active
to sterile neutrino oscillation signatures in the anti-neutrino energy spectrum
as a function of the distance to the core and energy. Performing a precise
search requires high light yield of the scintillating elements and uniformity
of the response in the detector volume. The SoLid experiment uses an innovative
hybrid technology with two different scintillators: polyvinyltoluene
scintillator cubes and $^6$LiF:ZnS(Ag) screens. A precision test bench based on
a $^{207}$Bi calibration source has been developed to study improvements on the
energy resolution and uniformity of the prompt scintillation signal of
antineutrino interactions. A trigger system selecting the 1~MeV conversion
electrons provides a Gaussian energy peak and allows for precise comparisons of
the different detector configurations that were considered to improve the SoLid
detector light collection. The light collection efficiency is influenced by the
choice of wrapping material, the position of the $^6$LiF:ZnS(Ag) screen, the
type of fibre, the number of optical fibres and the type of mirror at the end
of the fibre. This study shows that large gains in light collection efficiency
are possible compared to the SoLid SM1 prototype. The light yield for the SoLid
detector is expected to be at least 52$\pm$2 photo-avalanches per MeV per cube,
with a relative non-uniformity of 6 %, demonstrating that the required energy
resolution of at least 14 % at 1 MeV can be achieved.
