  PoGOLite is a balloon-borne hard X-ray polarimeter dedicated to the study of
point sources. Compton scattered events are registered using an array of
plastic scintillator units to determine the polarisation of incident X-rays in
the energy range 20 - 240 keV. In 2013, a near circumpolar balloon flight of 14
days duration was completed after launch from Esrange, Sweden, resulting in a
measurement of the linear polarisation of the Crab emission. Building on the
experience gained from this Pathfinder flight, the polarimeter is being
modified to improve performance for a second flight in 2016. Such
optimisations, based on Geant4 Monte Carlo simulations, take into account the
source characteristics, the instrument response and the background environment
which is dominated by atmospheric neutrons. This paper describes the
optimisation of the polarimeter and details the associated increase in
performance. The resulting design, PoGO+, is expected to improve the Minimum
Detectable Polarisation (MDP) for the Crab from 19.8% to 11.1% for a 5 day
flight. Assuming the same Crab polarisation fraction as measured during the
2013 flight, this improvement in MDP will allow a 5{\sigma} constrained result.
It will also allow the study of the nebula emission only (Crab off-pulse) and
Cygnus X-1 if in the hard state.
