How can I temporarily close an arched interior passageway?
I would take two sheets of plywood or similar material, cut slightly larger than the opening, place one on either side, and use a few big bolts &amp; nuts driven through to pull the panels towards each other. The tension from the bolts will hold the contraption in place; some weatherstripping or the like on the edges will create a better seal and protect your walls. You can fill the inside cavity with fiberglass insulation and paint the wood the same color as your walls.