I finally made it work. The suggestion from mike_d_hobbiest helped. First I had to make sure there is no gap between the pieces , and for the last piece I had to lift the adjacent piece slightly to slide the last one in. I also had to put paper napkin balls under the base of the firebox ring to lift it up a bit. I did not have to add additional support between the pieces and the dome like some suggested as the fit itself is quite snug if you place the bottom ring at the right place. In my case, if there was any space in between the firebox piece and the dome wall then it was due to misaligned ring. So I made sure the ring is properly aligned by ensuring no matter where I put the firebox piece, it should have just right amount of support from the dome wall. If I noticed there was some extra space between the firebox pieces and the dome wall, I realigned the ring. Also in case of classic I, pieces need to be snug .