marson wrote:
you should be able to order a solid core 24" door from a lumberyard.

And if not that, buy a 26 or 30" and cut it down.

Pay attention to what someone said about needed air. The return ducts
DO NOT supply combustion air unless they just dump into the closet vice
being plumbed into the ductwork as they should be. Without that you
are in for flameouts at best and a lot of cabon monoxide at worst.

Harry K