I went into a stove shop looking for a thimble to pass through a combustible wall into a masonry chimney. The previous owners...The ones that I bought the house from had a clay thimble with about 3 inches of space to a combistible wall. The stove shop I went to didn't have one and they'd have to special order it. The avalible space I have is 15"W and 16 "L. The guy at the stove shop said another option would be too take the metalbeatos pipe they sold and it could serve as my thimble and be cemented into the chimney. Just like my old one. That doesn't sound right. Can I do that?