/diy/ - Do It Yourself

My house is older and the previous owner converted the fireplace to a gas stove thing with the exhaust running up the chimney. I like it a lot, but now I've got a cat and a little 6 month old who crawls. I want to block off the space behind the stove so he can't get into the little alcove behind (and so critters can't sneak back out - I've seen a snail there once or twice) but obviously the stove gets hot. What sort of material should I use, and will I be able to cut it myself (no powersaws at my house, but my dad could bring some out next time he visits)? We were going to have someone out to reinforce the chimney due to some earthquake damage last year, is this something that sort of crew could handle, or is it simple enough I can do it myself?