  Challenging optical resonant gain achieved via the use of two geometrically
modematched Fabry-Perot cavities is anticipated in Any Light Particle Search II
(ALPS II) to extend the search sensitivity of light-weight sub-eV dark matter
candidates to uncharted parameter space of scientific interest. We report on
the precision optomechanics developed for geometrical modematching of optical
cavities. The general concept, the mechanical mounting scheme, and the use of
an autocollimator to streamline the metrology procedure are rigorously examined
using optical interferometry, all of which meet the requirements of ALPS II.
