I haven't built a lot of step through type of frames but my recommendation would also be to use twin laterals going from the head tube back to the dropouts as a top tube. Years ago when we were experimenting with frame types for our charity bikes in Ukraine, I made a step through with a straight very sloping top tube. It was a poor choice. The frame did not have enough stiffness and pedaling action caused the bicycle to sway. This is why step throughs with a single top tube are curved to attach higher up on the seat tube. I have a few curved top tubes if you want to go that route. However again I'm going to suggest (along with Andy) the superior design is twin laterals.
Last edited by Doug Fattic; 01-06-21 at 05:49 PM.