[dmun]

I like Ed's method of making the external mold out of an earthen form - I think your stacked method with cylindrical external moulds will leave you with thin and thick areas of your dome, and it won't heat uniformly. There was a recent discussion of a plain concrete dome. Why not do a test cast of one in portland / sand / gravel and see if it's manageable and if it works. You won't waste a lot of ruinously expensive castible refractory if it breaks turning over, and if it doesn't hold up after a few firings, you could bust it up and use it as rubble fill in your stand block voids. More workable than an earthen form would be a wet sand one in a kiddie pool, which you should be able to find on craigslist like your dogloo. Another advantage of the upside-down method is that you could leave the dogloo intact, and put it back up on craigslist when you were done with it.