I have always used commercial rosin cakes with our Violano.  It's rather
difficult to get them to fit the rosin tray, though.  In my experience
it is hard to cut them as they try to re-join together after the blade
has passed through.  You can't file it or saw it.  I tried making up
square trays from foil and melting the rosin into them (careful -- it
is flammable).

In the end I decided just to apply it by hand.  In the museum everyone
always wanted to see how it works with the top doors open, so I just
kept a small lump of it and applied it to the tops of the wheels for
about a second.  It is easy to over rosin the wheels, but you learn
that quite quickly.  You can explain to people what you are doing and
why.  They were always interested.

David Evans