Would it be possible for all of Germany to be within the Zollverein, under a more centralized German Confederation, but without Germany being formally united into a single state? This would mean that there are no internal tariffs within Germany, and Germany is more or less under the leadership of one country (probably Prussia), but all the states really retain their sovereignty.
Germany is defined as the territory of the German Confederation that made it into united Germany, plus the non-Confederation Prussian provinces. Luxemburg, Limburg, Schleswig, Alsace-Lorraine, and the Austrian territories are optional.