Here's a pic that better describes the problem...

I don’t remember offhand what that piece looks like — whether it has any milled ridges, etc. — but if it’s flat, maybe you could pick up a piece of aluminum bar stock at the hardware store and drill bolt holes into it to act as a replacement. Or two pieces with a spacer between, to hold the poly? I used a couple of those to support the ventilation fan I installed in my hfgh a couple years ago, and it was relatively easy to work with.

Another option might be just putting greenhouse bubble wrap over the whole vent while you wait for the replacement. You have another functioning vent, right?