I presume (you should confirm) that the USB port is not just for power i.e. is a full-function USB port, including data. This kind of port, for example, allows me to plug in a USB flash drive direct into the car (which for me is a way to play my entire music collection without using a smartphone anyway).
I have a cigarette lighter port to USB charger adapter, which works well for me. The charger has two outlets - USB-C for the Librem 5 and USB-A for e.g. an iPhone. Just make sure to get one that provides enough power on the USB-C port while still leaving enough power budget for the USB-A port - and of course also check the power rating on the cigarette lighter port itself.