Hello, i wanted to put a cover on opening of 33

<p>Hello,</p> <p>i wanted to put a cover on opening of 33" x 23" on my unused fireplace as it has draft. I noticed the flue even is close has a gap so that might where the draft came from.</p> <p>I am thinking to use this but i am unusre if its environmentally friendly?</p> <p>https://www.lowes.ca/foam-board-insulation/dow-cladmate-4-ft-x-8-ft-extruded-polystyrene-insulation_10291799.htmlThanks</p> <p><br />Should i just use a plywood and paint with anti-mold+mildew interior paint or use a pressure treated plywood?</p> <p>I will not use this fireplace anymore.</p> <p>Thanks<br />Clayton</p>