Floor/Base insulation - building an RMH on the bare earth

I am building my first RMH in a 10by20 shelterlogic tent/portable garage. which i will then be living in. obviously i need to insulate the mass from the ground in order to retain heat. I have decided to use Styrofoam board under the mass/bench though it is a compromise, since it is a bad choice environmentally. I figure i cant use foam board under the burn tunnel because it would melt. so i am going for the standered brick perlite/clay mix under the burn tunnel according to the book (Ianto)

Question; does this sound right too you guys? could i use foam board under the burn tunnel if there was a layer of brick between? how much heat will i lose too the ground if i don't? anyone found a better way?

appreciate any feedback, this is my first post, Thanks