I used a actual Speed Trap to locate both my belly hanger and my bill, and the bill angle and size. The closer to the original you can get your bait, the better it will run.
I am not proud. I used the hard work of the original bait designers as a template, first. Then, as I tried different copies, I learned what made that particular part of the design work.
For instance, the triangular head and front shoulders of the original is what makes it so stable even when it's burned on the retrieve, but it took me four versions to figure that out. I wasn't sharp enough to pick up on that at first. Same with the tail shape and taper, and how the bait is shaped like and arrow head, with the widest part back past the mid point.
Because I was building solid PVC and balsa baits, instead of a hollow plastic bait, I had to figure out both the amount of ballast, and the moving rattle ballast location and amount on my own, but I had already learned roughly the right placement from previous cranks I'd made with the same rattle setup.
The Speed Trap is a crank in which each little detail truly serves a purpose. I bet the little wrinkle at the top of the tail probably does something, too, but I'm not smart enough to figure that one out.
Those guys at Luhr Jensen really nailed it!