Tom W1EAT provides a schematic for his mod of a variable IF bandwidth circuit for the µBITx. The RX and TX voltages are about 12V, so he used a resistor network to keep the applied voltage to about 1 to 9V because that is the usable range for the varicap diodes he uses. As the voltage on the caps is reduced the higher audio frequencies are reduced until the bandwidth is probably 100Hz or so at 1 volt. The filter centre frequency is very low, 200 Hz or so with Tom’s BFO setting.
The BB112 has about 500pf capacity at 1 volt applied.
The 4.7K resistors that feed the voltage to the VVC diodes could be 10 or 20 K, or what have you, as there is very little current needed.