That would be the best way to get the top of the riser not too high above the main floor. If you can dig down to set the core into the floor, why not just go farther so you can have a decent height to the feed tube to be able to use standard
firewood?
A 1:1.5:3 ratio would give you a 16" deep feed, 24" burn tunnel floor, and 48" riser. This would yield a 34-36" high bell top if metal... not coffee table height, but you couldn't use that surface as a coffee table anyway, it will be too hot for anything but cooking. For a 6" system, you wouldn't need more than about a 14" diameter section to be tall; the rest of a coffee table/bench could be whatever height you want. To make the tall section comfortable to be near, you would need to add masonry walls around it giving a total diameter around 22" or so. A masonry screen with gaps for airflow could be thinner and still be comfortable.
If your chimney has a reliable strong draft, you could shorten the riser a few inches and still have it work well. My 8" J-tube has 16"/24"/45" dimensions, and with my constant positive draft chimney it runs beautifully. My
experience with peepholes to watch the tops of rocket risers is that the flames in a finished system rarely reach the top of the riser at 45", so combustion is complete and you don't need more height unless the draft requires the
boost.