There are lots of ways to reduce the risk of smokeback. Trouble is, many of them just end up increasing the risk of 'smoke forward' - inefficient, polluting burn.
The 'conventional' solution in
RMH's is to increase the heat riser height, and/or shorten the burn tunnel, until things balance well. You want constant, good draft; but you want to also maintain good mixing and complete consumption of fuel.
The right angles in the burn tunnel help mix the gases while in the hot part of the flame path for complete combustion. There have been a few stoves that use a gentle slope, e.g. Lorena stove, Good stove.
In a RMH I would expect a gentler slope to have a similar effect to a rounder angle at the heat riser join: smoother flow, less mixing, more smoke out the top.
Please try it, though, since you've got the test bed and the idea. See what happens. It might be a great option. There may be an angle that still gives adequate mixing, or a way to do a 'smoke shelf' or 'smoke throat' that replaces the lost corner with a fin-like vortex shelf.
Adding undue complications to an experiment with a much simpler
answer ...
I wonder if you are using gum fuel-wood, if it will burn rich or pitchy, like our pitch-pine or madrone. Ianto's ash-trap under the fuel feed can be useful for certain dense or pitchy fuel woods, as it provides a way to get longer fuels completely into the feed without waiting for their lower ends to burn down. Wet
wood, and pitchy wood, both tend to burn up the outside before the ends collapse. The extra space only lasts for the first little while until coals/ash build up, but this can be enough to heat things up and get a good draft going ahead of the flame-creep.
(Good, dry, seasoned fuel wood is always preferable. But in a rainforest you're going to have more than 15% moisture because the air itself is above 80% most of the time. And you have what you have as far as local fuels. You can balance pitchy with punky or scrap to a certain extent, but sometimes you can find a work-around that suits the local norm.)
I wouldn't mind seeing a coal-fired RMH; they have coal-powder briquette tea stoves in Japan that are pretty cool. But a friend doing early calculations in the late 90's told me we have maybe 50-80 years of coal left. I hope we have wood much longer than that. I want us to learn to use it well, so that we still have it.
A lot of places Ianto went to help improve their 'primitive' kitchen fire technology, they had a special trick, and if he was very good they would show it to him. A squirt-bottle of kerosene, or really stout matches, or parrafin-and-sawdust firelighters, or a conduit tube to blow air through. Ernie used to carry a scrap of car tire in the Arctic for unquenchable tinder. At the edges where it matters, people appreciate the benefits of fossil fuels. Even where they are expensive, a little goes a long way. If you don't have bitumin, tar, coal, whale-oil, wax, etc, you get really good at spotting fat-wood and fire-mushroom and all the other useful natural boosters.
-Erica