My masonry flue is about 8.5 by 9.5 inches and has no clay liner. I know it needs cleaning before I install my flexible liner, so I bought an 8 x 8 steel brush. I was planning to use flexible poles to clean the lower part where it's not straight then do the rest using a ropes. But then I was reading the package for the brush and it was saying how important it is that the brush be the right size to exert adequate force. I don't think anyone makes a brush to fit an odd size like mine. Any suggestions or advice before I proceed. What would a pro chimney sweep do?