In the book "Wide-field astrophotography", by Robert Reeves
, chapter 16 page 417 there are an explanation how to built and use
one of this apparatus . I made one and it works great.
best
Jose Carlos Diniz
I should mention that the one I built that is
shown in the book uses an 8-inch diameter rear view spot mirror off an
18-wheeler truck tractor. These mirrors don't give a full 180 degree
field of view (closer to 140 to 160 degrees depending on how wide-angle the
mirror is, but the mirror surface is much smoother than the chromed sheet
metal of a baby moon hubcap.
Both Chris' and my all-sky mirror setups were in
the same article in Astronomy about 15 years ago.