rmh garage

Hi everyone, this is my first post.
I've been listening to the survival podcast for about to weeks and the ideas are swirling in my head.
I hope this post brings a fair amount of discussion
I have a detached block contructed garage with a pit (filled in due to insurance purposes)
I want to build a RMH and need some guidance.
I was thinking of using the pit to run the heat pipe through and back filing it with sand and broken concrete I have laying around as the thermal mass.
Thanks and look forward to responses, and learning from what this community has to offer.

RMH very slow even heat.

typical garage requirement is fast warm up for use.

30 degree garage might take a full day to provide desired warmth.