You could use the flowerpot idea for something to warm your hands by, but it's not magic. It won't heat a whole space, just a small area right next to it... two candlepower is still two candlepower, the pot just keeps more of it near the source instead of dispersing to the whole room where it would be undetectable.
As you say your soil is very clayey, it should make strong and durable cob. The heat of the fire will actually turn the inner parts into pottery, which will last pretty much forever unless you get a serious inferno to melt the inner face (not likely).
You do need a decent chimney for an RMH to work well; flexible duct is too rough for good flow. A few feet of steel duct or stovepipe will work well, and with the low exhaust temperatures can be safe going through the polycarbonate. I would give it some metal flashing just to be safe. The 4" batch box in the link above would only need some 4" duct for a chimney. I posted some pictures of a cob rocket cooking stove I built; you can see how simple it can be.
https://permies.com/t/52509/Clay-Rocket-core-Bell-RMH#428887