Hello Ryan,
The following thread has a build by Kirt Mobert where he uses wood ash as a natural refractory mix
Batch rocket
Kirk said: "I think i should point out that the firebox of this stove is a homemade mixture that consists of crushed red brick (grog), wood ash, cow manure and locally sourced clay soil."
EDIT: The first few pictures shows how the stove is raised off the floor before the wood ash mix is used.