  An ultrastable optical reference cavity with re-entrant fused silica mirrors
and a ULE spacer structure is designed through finite element analysis. The
designed cavity has a low thermal noise limit of $1\times10^{-16}$ and a
flexible zero crossing temperature of the effective coefficient of thermal
expansion (CTE). The CTE zero crossing temperature difference between a
composite cavity and a pure ULE cavity can be tuned from $-10\ ^{\circ}$C to
$23\ ^{\circ}$C, which enables operation of the designed reference cavity near
room temperature without worrying about the CTE zero crossing temperature of
the ULE spacer. The design can be applied to cavities with different lengths.
Vibration immunity of the cavity is also achieved through structure
optimization.
