I have the Altoona one it is very nice, and I believe he can build it and paint it for you if you like. I gave used the Walthers HO chimney on a riser with good success. No riser needed if you mounts on a roof top. But if on the same level as a structure, simply make a riser out of any number of items to sakes the total height.
Square is simpler than round. I used a large square dowel covered With plastruct brick sheet, also I used small back splash tile from Home Depot to make the base, it looked like stone blocks.