  We describe a compactification by stable pairs (also known as KSBA
compactification) of the $4$-dimensional family of Enriques surfaces which
arise as the $\mathbb{Z}_2^2$-covers of the blow up of $\mathbb{P}^2$ at three
general points branched along a configuration of three pairs of lines. Up to a
finite group action, we show that this compactification is isomorphic to the
toric variety associated to the secondary polytope of the unit cube. We relate
the KSBA compactification considered to the Baily-Borel compactification of the
same family of Enriques surfaces. Part of the KSBA boundary has a toroidal
behavior, another part is isomorphic to the Baily-Borel compactification, and
what remains is a mixture of these two. We relate the stable pair
compactification studied here with Looijenga's semitoric compactifications.
