Am building a modle gazebo and having trouble with the roof which is an octagonal inverted cone. The drip line must cover 12.5"(diameter). In order to get the look I want the triangular pieces must be 10"-11" base to peak making a very high and steep pitch. How do I cut these triangles and especially the beveled edges to accomplish this? Of course the floor is also octagonal therefore, a matching roof. Any help will be greatly appreciated.