As all sides of the triangles are tapered, the bottom course of the built dome isnít flat, itís tapered. Also I found the bottom wavered around about 25mm from level. To straighten it up, I turned it upside down, put a ring on top of it and measured down at the largest gap. I tied a builders string around the dome, tight, and pushed it down to the same distance from the ring, all the way around the dome. Iím hoping this gives me a straight line. I drew the line by running a marker around the string.
I then made up a small hot wire cutter, using a mini hacksaw handle and taped a builderís level to the top of it. The builders level gave me some idea of the angle the cutter was sitting at. I got the wire hot using my home brew dew heater and ran it around the dome. As the hot wire was very short I could get it pretty hot which makes for a quick cut, plus it was hot enough to cut through the glue in the triangle joints. Red hot is too hot, that melts holes in the foam.
If you look at the second dome image you will see why I had to do this twice. This was initially the lowest point, so I got the rest of the dome level with this point, but this point still had the triangle taper on it so it was not flat. I dropped the string line down another 10mm, drew a new line and cut the dome again. Success !!!!
This action will change the outer diameter of the dome slightly. If you are after a tight fitting setup, Iíd do this and measure before cutting the rings.