As most slave clock coils are around 3 to 4 ohms. The easiest option is to make up a string of 3.3 or 3.9 ohm, 5 Watt wire wound resistors in series with the slave dial circuit.
In the case that you remove a slave dial add an extra resistor in series in the case that you use a 20 Volts dc supply.
You can leave the R string in series permanently and short or unshort a resistor out to suit the series circuit.
Try to source a 60 or 100 ohms 5 or 10 Watt wire wound pot meter which gives you the complete ohms range,
A second hand shop of old electronic gear may have them with a bit of luck.
or
e.g 3 slave dials at 250 mA ( 4 ohms ea) and on 20 Volts dc require a series R of 80 ohms minus 12 = 68 ohms.
You could make a string of 20 x 3.3 ohms resistors ( makes 66 ohms)
or fit two 3.3 ohm resistros and one 47 ohms or 56 ohms R (5Watt) to allowe you to add one or two extra slaves if required, less messy wiring.
or probably better to opt for a lower voltage, say 12 Volts.
I run a Gents master of 12 Volts with one slave dial ( 4 ohms ) and a series R of 39 ohms, about 270 mA and seems to work perfectly with no missing clock pulses.