When I’m feeling lazy and need to drop a few volts I just put a few rectifier diodes in series. They will drop about .7 volt each so 5 gives you enough voltage drop to run from 12 volts where 9 is desired.
I did this a couple times with my DVD player which has a 12 volt lighter socket car adapter power cable and I have a very large 16V external battery pack for computers. Could not find a spec for maximum input to the car adapter but figured it must be safe at 14.5 volts as that can be expected on a vehicle so used four 3 amp rectifiers. Worked fine and I watched movies all the way from the US to Thailand but the rectifiers got so hot they melted the plastic body of the car adapter I had taped them against.
Next trip I used a 3-Terminal adjustable regulator (don’t recall which one, one of the LM3XX series) on a large aluminum heat sink and again the heat sink got very hot but everything worked.
Would like to find a small switching regulator module so as to not drop the extra voltage as heat. But then again not sure they would let me bring such a large battery onto a plane these days. It is about 5 in by 9 in by an inch thick.
A related and funny story was the time I forgot my laptop power brick when travelling to Thailand (my wife is Thai – we go often) but had my airplane power adapter which takes in 12 volts. So was walking around Bangkok and spotted a electronics store but only 12 volt power supply that had was huge and weighed probably 15 pounds. So had them wire up a lighter socket to it and used it the entire trip. Made a few flights with it as carry on that trip and while it raised a few eyebrows it was allowed on the plane. But that was in 2000.