Help: base and hearth design

[jmilc]

I'm considering a base design that includes a 6 mm galv steel plate for structural support, (positioned into the bricks 2/3rds of the way up the brick wall) then 30 cm of compacted gravel followed by a 10 cm insulation barrier of either a mix of cement and garden vermiculite or insulation bricks that I can purchase that were demolished from a kiln, then standard brick pavers or solid building bricks (can't get firebricks). Can someone advise whether this will work? The steel plate shouldn't receive any heat, and wondering which is the best insulation layer to use - the kiln bricks or a wet garden vermiculite and cement concrete mix. I also have a bag of cement fondue, should I use this as well either side of the insulation layer or reserve this for mortar over the brick dome? Thanks