I searched high and low at the Carlisle swap meet for something that would work...
I came home with some 55 Chev bushings that are a larger diameter. I trimmed them down, but they are too thin and leave too much clearance. :P
Out of desperation, I settled on some steel 3/4" EMT conduit. Took a lengthwise slice in them to reduce the diameter to fit. The z-bar now fits nicely. I don't like the steel-on steel contact, but it'll do for now.
Still looking for the correct bushings if someone knows of any....