If the border tax is just for countries without a similar tax, let's consider one example, say steel. Foundries in countries that fall under the taxed regime then have an incentive to produce the cheapest (and dirtiest) steel, since they get no benefit vs the other foundries (you could rightly retort though that this is largely the way it is today). Maybe the tax then should try to evaluate the relative carbon intensity of each supplier, rather than just by region/country, but good luck avoiding cheaters around the world. I guess overall I have a hard time imagining that you could create a fair tax/duty for carbon intensive industries that accounted for all of these potentially perverse incentives unique to each industry (just take a look at the US tax code), given how hard it is to write and pass policy without succumbing to special interests.