Meant to say octagonal shaped room, lol. What I do is make a cross shape with the normal foundation tool. So, you'd put down three squares of foundation in a row, then one more square on either side of the middle piece. It should look like a plus sign. Then switch to the diagonal foundation tool. Drag it from the uppermost corner of the cross to the opposite corner, then pull it out so that it makes a piece of foundation that's 2 x 1 squares. It should have filled out the corners of the cross. Do it to the other side, and it should make an octagon. Sorry, it's hard to explain... If you play around with it for a little while it should make sense. Someone more experienced with building might be able to give you a better method, too.