Laptop power supply from 12V source?
The DC-DC converter route is, by far, the simplest and most efficient solution.

That being said, with DC-DC's you do tend to get what you pay for. A nice CUI or MuRata (name brand) DC-DC likely will keep your laptop safe. As you noted, off-brand products, may need some sort of filter on their output to avoid voltage spikes.

The issue you are seeing with the potentiometer is likely because they used a dirt-cheap and low-grade pot, with poor linearity or a dirty wiper. Once you adjust it to the right voltage, you can use some stiff epoxy/glue to lock it into place, and it should be fine.

For the crowbar circuit, a 20V Zener diode and 6A fuse are what you'd want. That's the simplest, albeit not great; it'll still allow voltage spikes, but at least keep them contained to about 24V. A large capacitor (the DC-DC converter should list the maximum capacitance it can tolerate in its datasheet) would help, too.

One final consideration: the high-power laptop chargers often have some level of intelligence to them. My HP laptop, for example, knows when I plug in a non-OEM charger, and complains about that; it will still charge, but at a reduced rate.