You want higher voltage out than in at significant power. This will require some sort of switching power supply in practise. You also need significant energy storage to ride out the times when there is no input power.
The correct approach is likely to step back two levels and think about the problem differently. This looks like you are asking about a detail of some imagined solution where what is really needed is a better approach.
However, to answer the question directly, I'd probably make a switching power supply that charged a 12 V car battery when input power is available. The output is the car battery output directly. As long as sufficient current can be drawn from the intermittent input voltage, this system should be sustainable long term (the battery will not be discharged on average).
Note that the switcher will have some inefficiency, so more input current is needed during the input bursts than you might otherwise think. The output needs to deliver 12 V x 2.1 A = 25.2 W. Since the input is on half the time on average, a total of 50.4 W is needed when it is on due to power conservation alone. Let's say the switching power supply is overall 80% efficient. That means the input must supply 50.4 W / 80% = 63 W when on. The current that the switcher will draw from the input during the on phase will therefore be 63 W / 11.5 V = 5.5 A.