Is there a commercial shoe isolator that will provide the center
terminal only for connection? Or do I have to build one?

A well-stocked photo store should have one. The shoe and central terminal
connect to a short lead with a PC connector. (I bought one a few years ago.)

I'd recommend covering the bottom of the shoe with two or three layers of
Magic Mending Tape, then cutting out the area over the central terminal. The
tape keeps the extra pins from contacting anything.