Scott Baret makes some good points.

If you are protecting your Mac from sunlight (which you should), just close the curtains as a starting point. Put a thick cotton covering over the precious Mac (an unworn pillow case or sheet will suffice) and then seek out some dense black plastic sheet to put on top. You need the cotton sheet so that the plastic sheet is not in direct contact with the Mac case.

By chance, my Mac collection is on the west side of my house which warms up more slowly in spring and summer. In winter it is close to the central heating system. Thus my Macs don't experience too much temperature variation -- or they don't suffer extremes. They do suffer from the threat of a drenching from water cisterns, thus my tanks are modern and checked. The Macs would survive exposure to clean water but not the manuals and magazines and boxes.