Hi - Welcome to the forum!
There are a number of ways to do this. The sloping edge sections make it more difficult but I think the easiest way is to cut it out of a piece of floor. Probably sounds odd but the floor tool is very flexible and you can create almost any shape and make it re-useable in future projects.
Start a new project and use Floor By Picking Points to draw the shape you want to the size you want, then set the thickness to the depth of the chimney. When done, use the View Filter to turn off the Terrain then do a File / Export / 3D Model and choose 3DS. Give it a name and save.
Now go to HVAC or Interior Accessories, right click on one of the existing elements and choose Add New Element. In the dialog that pops up choose "Import geometry from file" and browse to your file (make sure you are searching for 3DS - defaults to DXF). When you import you can size and apply materials if you want or you can do that later.
The image below should help and the PDF in the following post has the details how to do it:
http://forums.turbofloorplan.com/index.php/topic,1130.0.html.
The rendered image is the floor cutout seen in the first image.
Allan
