Ok, my bad, those were not diameters, but circumferences of the helix. I used the same formula that you showed to find those, but could not find the website I found(using a different computer now). And I even copied the numbers down wrong. The inside circumference should be 3
4.8 inches. After putting these numbers into Inventor I got this shape:
After printing this out and making a scale model out of paper, I think this shape will work well enough. Thanks for your help!
