Thanks for the ideas for blocking the openings. I made my blocks out of 3/8-inch plexiglass which I had left over from another project. The blocking element is made of two pieces screwed together, and a "T"-shaped piece holds the block against the I-beam and allows it to float up/down and left/right. Time will tell if I allowed enough room for movement.
The plexiglass pieces were epoxied together. The screws are stainless steel and I tapped threads in the plexiglass to accept the screws. (The extra horizontal piece glued at the top serves no purpose. I can't remember why I thought it was needed 6 months ago. I made the first block last fall, but didn't install it until now.)
Pictures show "before" and "after".