You can even IMPROVE on the flat roof by having a curved roof. All you have to do is trim the top of the piston to match the curved roof. A straight line is the shortest distance between two points - if you take a curved line between the two points, you INCREASE the distance between the two points. - In other words, you increase the size/length of the opening when it first opens.
That will give you an even stronger pulse to work with in an expansion chamber. However, we are trying to eliminate the short-circuiting on this jug, which is entirely a different problem.
We aren't working with an expansion chamber and tuning the exhaust, we are working with FLOW. We need to correct the flow so that the intake mixture stays in the jug to be burned.