@Fox James: Thank you for that thread, that basically answered all my questions
Yesterday evening I dry-stacked the core and did a test run. Went well I think, the fire was very rockety and the heat out of the chimney was intens. Very exciting to see the power of the core.
For this 8" system I ended up with the following dimensions:
Woodfeed: 20x20cm opening, 41cm height
Burn tunnel: 20x19cm (WxH), 60 cm long, roof of 20cm
Heat riser: 20x20cm opening, 119cm height.
I made the heat riser out of 4cm thick firebricks because i bought a second hand batch and have too many anyway. I plan on insulating with rockwool, which i have laying around. The heatriser I was thinking of 6,5cm thick, and the woodfeed/burntunnel with a double layer of 4cm thick. And then finish it with cob.
Next step will be to cut the half barrels and clean them a bit more. I'm also gathering some materials and soon i will start with tearing down the old chimney to make room for the RHM.