Surge protector for car USB port
If "car manual" says so about USB sticks, it is a very poor design. USB ports are fed from 5 V supply, which should be generated from inside of USB-enabled equipment. 12 v to 5 V should provide enough margin to have a stable +5V with a little engineering effort. So making a L-C filter makes no sense.

The problem is likely in the fact that 12V rail fluctuates wildly when cranking the car engine. Here is the cold-crank waveform as assumed by ISO 7637-2 standard



So it is expected that the supply voltage drops down to 3V level and stays at 50% level for up to 20 seconds. It is difficult (costly) to maintain an audio/USB/on-board computer equipment to sustain such deep and long drops. So in most cars (and especially inexpensive ones) the equipment simply is turned off during cold cranking. The problem with USB is likely that "surprise disconnect" of USB host might cause device corruption if there is no sufficient power hold and graceful system shutdown and recovery. A L-C filter of any reasonable size cannot help here. The problem can be solved by having a small secondary (backup) battery that is capable of maintaining quality power to USB-related equipment just for 20-30 seconds, a small local UPS so to speak.