"In summary, the “separation” phrase so frequently invoked today was rarely mentioned by any of the Founders"
Seems I found several just by Madison, the author of the first amendment. So rare you can only find a few dozen?
Here's a blurb from the same author about James Madison:
"James Madison trained for ministry with the Rev. Dr. John Witherspoon, and Madison's writings are replete with declarations of his faith in God and in Christ. In fact, for proof of this, one only need read his letter to Attorney General Bradford wherein Madison laments that public officials are not bold enough about their Christian faith in public and that public officials should be "fervent advocates in the cause of Christ." And while Madison did allude to a "wall of separation," contemporary writers frequently refuse to allow Madison to provide his own definition of that "wall." According to Madison, the purpose of that "wall" was only to prevent Congress from passing a national law to establish a national religion."
Don't get hung up on a word. Look to how the word is used.