One thought I had Mick for making the dome is to compact wet sand form the shape and then build the dome Scoop out the sand when you are done. This is a popular method used when making a cob oven. The natural lava rock here can be found in brick like chunks that have some consistency in shape and size. They stack quite nicely. I does take sometime though to aquire said rocks. If I still lived out of town wouldn't be a problem, had 5 acres of great rocks!