If this is a temporary situation then you might conside using plastic wrap like the kind they use to wrap shipping pallets. You would need some kind of frame (got any old telecomm frames or unused shelving?)

Another option which we use for hot / cold aisle isolation is freezer curtains like to see in a grocery store freezer room doorway. They are fire rated to be non toxic. They are not terribly expensive. You need to suspend a metal bar for them to hang from. You can walk through them as needed.

Hope some of these ideas are helpful!

I had this issue in my data center and was able to get my HVAC maintenance vendor to fabricate a piece of sheet metal that I fastened to my APC racks on either side of the opening using the holes and screws used to bay the racks together. They painted it black to match the cabinets and it looked good until I replaced it with a tape library. Good luck.