Sure, it's here.
FYI, Madison's letter is over 1,700 words in 17 paragraphs, so can't be read in one quick glance.
But it remains the most explicit expression of our Founders' original intentions regarding an alleged unlimited "right of secession".
Madison's letter to Trist was written 54 years after the Declaration of Independence, too long after to be considered contemporaneous with the nation's founding.
In the letter to Trist the former President wrote candidly about his advanced age and cited it as a reason his end-of-life musings could not be trusted.
If, as you say, this private letter “remains the most explicit expression” of the view that the Constitution forbids secession, then Brother Joe your Mother Church has a weak foundation indeed.