Ivory keytops were originally glued using hot animal glue.  Add
titanium dioxide (from an art supply house) for a white tint as ivory
is translucent.  Do this on a warm day and/or heat the glue surfaces
to make it easier.

You can buy bronze glue cauls from piano suppliers which are heated,
but spring clamps with squares of wood work if you're careful.  Also
available are glue "wafers" which are immersed in hot water for use.
These are pieces of cloth coated with animal glue and white tint.

Philip Jamison