Definately leave out the portland.
Use a high temp clay, I prefer Kaolin, which runs $10-$15 for 50lb, use silica sand or grog, approx 20lbs grog/sand to 50lb of clay, to reduce shrinkage and add strength and if you want it to be insilated add in some foam beads, bean bag chair filling works good, add as much foam as your mix will allow and still hold together, add water very little at a time and mix well. Then do 1/4"-1/2" hot face, [clay mix with no foam in it].
Let it dry slow and then fire it slowly, if you fire it too fast it will crumble.
It will handle 3000F with no problem.
I use this mix for my forges and casting furnaces.
welder19