Perhaps someone in the group will have a circuit for a regulated 12V
charger.

If not, I would experiment, starting with something simple. Stick a 25-ohm
resistor in series with the output of a 12V supply and see what happens.

If you want true constant-current charge, you'll need a higher voltage (25V,
perhaps?). You might, for example, put the battery in the collector circuit
of a power transistor and vary the base bias to get the charging current you
want.