When you cut groves for the spirals, cut them at an angle
Look for my articles on how to build a home HT oven,
there're links to pics there.

Typical oven brick is very light & soft, you can literally
make those spirals with yer fingernails.

It is UBER important for the brick to be a good insulator -
with inside temp in 2000F are, you should be able to touch
the outside surface and not get burns.

Casting your own using refractory cement won;t be nearly
as insulating as those bricks they sell @ Ceramic supply
stores.