Our Founders thoughts of what the Union is is extremely important since they came from the principle instigators of the CC and the Constitution itself.
The Federalist Papers had no legal significance, either. They provided the basis for understanding the document. John Marshall said that when he approached any case the first thing he did was consult the Federalist to see how to think about it.
These essays (the greatest political thought of their and our day) were written by a couple of random guys named James Madison and Alexander Hamilton with a couple by John Jay. Hamilton wrote two thirds of them. George Washington believed the same thing about secession and explicitly warned about it.
There are plenty of implicit supports for an indissoluble, perpetual union. The Articles stated that clearly and its’ child, the Constitution made that indissoluble Union even more perfect.
You can’t even get a divorce without legal sanction (unless you live in a Islamic state), how much more necessary is it to dissolve a perpetual Union?