If the 12 volt battery is just for the BB and powers nothing else that could tie grounds together, a single chip voltage regulator could be cobbled together. Would need to pay good attention the polarity at the connector as the barrel tip needs to be negative. The LM7809 is a ‘positive’ voltage regulator chip and there is an LM7909 which a ‘negative’ voltage regulator chip and the battery and output polarity would be flipped on the diagram. Either will handle up to 1.5A with heatsinking.
Trial by fire… learn to solder 
