Iterative designing is what 3d printers are for. Print your design out, test it, fix the design, repeat. Sometimes you get it right in a few tries, and sometimes...
modular loom kits that snap together but are missing a concave "L" so she challenged me to make one for her.
I used the technique that I've talked about here and in my book, the poor man's scanner, wheteby the object is photographed and the image is used to insure proper proportions. The first designs were made flat on the side so they could stand up but that was quickly abandoned as the holes weren't as accurate that way. Then the whole thing wasn't beveled properly, them it was to small, then to large. Finally I got one that was close enough that I could make it work with just a little sanding.
Now that it works I'll be uploading it soon, even though the final design isn't perfect. It's good enough that I got mine, so that's good enough for me.