Steve:
When I took my car apart I found a small rubber washer/grommet surrounding the wiper shaft where it passes through the windshield frame on the outside (chromed) surface. This rubber piece was underneath a small chromed washer that was in place under the nut holding the wiper in place. Upon reassembly I modified an appropriately sized rubber grommet to fit.
A400 Guy
You could fabricate a spacer tube, but I don't really think it's necessary. My guess is that you'd probably strip the wiper shaft threads before you collapsed the windshield frame bracket. Also given the size of the holes in the frame and realizing that you will need a spacer tube with a larger OD than the hole, you may find that you end up with an unwieldy assembly. Just my thoughts .... Let us know what you ultimately decide to do.