  A new generation of interferometric instruments is emerging which aim to use
intensity mapping of redshifted $21\,$cm radiation to measure the large-scale
structure of the Universe at $z\simeq 1-2$ over wide areas of sky. While these
instruments typically have limited angular resolution, they cover huge volumes
and thus can be used to provide large samples of rare objects. In this paper we
study how well such instruments could find spatially extended large-scale
structures, such as cosmic voids, using a matched filter formalism. Such a
formalism allows us to work in Fourier space, the natural space for
interferometers, and to study the impact of finite $u-v$ coverage, noise and
foregrounds on our ability to recover voids. We find that in the absence of
foregrounds such instruments would provide enormous catalogs of voids, with
high completeness, but that control of foregrounds is key to realizing this
goal.
