  We consider a model of Bose-Einstein condensate of weakly interacting
off-shell gravitons in the regime that is far from the quantum critical point.
Working in static spherically symmetric setup, recent study has demonstrated
that the effective spacetime geometry of this condensate is a gravastar. In
this paper we make three generalizations: introducing a composite of two sets
of off-shell gravitons with different wavelength to enable richer geometries
for the interior and exterior spacetimes, working in $f({\mathcal R})$ gravity,
and extending the calculations to higher dimensions. We find that the effective
spacetime geometry is again a gravastar, but now with a metric which strongly
depends on the modified gravity function $f({\mathcal R})$. This implies that
the interior of the gravastar can be de Sitter or anti-de Sitter and the
exterior can be Schwarzschild, Schwarzschild-de Sitter, or
Schwarzschild-anti-de Sitter, with a condition that the cosmological constant
for the exterior must be smaller than the one for the interior. These
geometries are determined by the function $f({\mathcal R})$, in contrast to
previous works where they were selected by hand. We also presented a new
possible value for the size of the gravastar provided a certain inequality is
satisfied. This restriction can be seen manifested in the behavior of the
interior graviton wavelength as a function of spacetime dimension.
