To pull a dome that large is going to take some specialized gear + custom template. I don't know if the guying blowing domes do them that big or are able to keep them symmetrical at that size.
DIY large dome on the cheap.
You will need:
1. Weber BBQ (kettle style) and heat beads.
2. Large square of thick plastic sheet of at least 6mm - 10mm that is about 20% larger than the dome dia.
3. 12" (12.7mm) Ply-Wood template with hole pre-cut out at the same size as your desired dome.
3. Nuts and bolts to attach the plastic to one side of the Ply-Wood.
4. A jig that will allow you to suspend the wood/plastic over the BBQ and also allow you to move it away quickly.
Steps:
1. Light the BBQ and let the beads get really hot (no smoke). Adding 6 heat bricks could get you in excess of 400 degrees C. You only need between 120 and 180 on the surface, but more due to both the area and the thickness.
3. Position the wood/plastic over the BBQ about 3 feet up.
4. As the plastic heats to the ideal temp, it will slump into a perfect dome. Just be careful not to over do it and you may have to raise the plastic as it slumps so it does not touch the BBQ.
And that is about as cheap a way to get the size you want.