This question fascinates me. Present build will feature a riser that starts out square, morphs to a wider circular shape, then narrows to a circle that has the same cross sectional area as the initial square. Also putting together an equal height riser that is just circular all the way, for a comparison. What I've read about pulse jets and two stroke expansion chambers makes me think that the more complicated shape might really offer advantages as far as when the burn will reach high temperature and complete combustion. Wish me luck, this is going to involve some complicated cardboard forms and fancy ceramic/vermiculite work...🙏