The rundown: Old building, Bathroom fixtures mounted to plywood and
tile cut around their footprints (why? I have no idea). I will be
replacing fixtures, but mounting on top of tile this time, so I'll
need to fill in discrepancies in the footprint. I have not been able
to find matching floor tile, but I do have a couple pieces that should
be sufficient for visible areas.

Someone recommended that I use quick-setting concrete to fill in for
1/4" thick tile -under- the fixtures (not visible). Any
recommendations on brand/type of concrete? It would be great if it set
in a couple hours.