Perlite Cob Mix for Rocket Mass Heater

I'm trying to plan out a rocket mass heater for my house. My house is two stories with a basement. I've seen people using a perlite cob mix for the insulation layer beneath the rocket mass heater. Can I use the perlite cob for the bench of my rocket mass heater assuming that it isn't used as an actual bench. I know that the perlite does not have enough compression strength for that. What I really am trying to do is lighten the load on my wood floors. I'm still planning on Shoring the structure up. Any help and input would be great. I just want to make it as light as possible to reduce the stress from the heavy load.

As you need the mass for thermal storage, can you build it above or in a wall?
Regarding the stability of clay/perlite: As long as you cover it with a thin layer of normal clay (5mm), it is incredible strong. Unless you want to put a grand piano on it, I don't think compressive strength would be an issue.

The perlite is mainly for insulation, so it's good around the burn tunnel and under the mass if you want to isolate it from the floor, but definitely a bad thing to have in your mass, as it reduces the heat-absorbing ability. Can you use a brace underneath the mass?