I think I found a workable solution to connecting cables with straight banana plugs to the down-facing binders on our Thiels.
Rummaging around through my audio junk box I found a set of four gold spade lugs I had bought off Amazon. These are very thick, fit a 5/16 post, and have a wire barrel that just happens to fit a 4 mm banana plug.
https://imgur.com/gallery/t199uNz
These aren't the ones I bought but I'd probably choose these instead--being Nakamichi.

