Because my weights kept coming loose during a workout (can you say ANNOYING) I was actually in the process of designing the same thing (pictured in purple). I kept failing in my attempts because I was trying to round the values to units that actually made sense.
As mar363636 said, for some reason weights use a weird thread measurement all their own.
Since this description seemed so familiar to my own findings I took a risk and printed this thing. (PLA, 3 shell, 10%infill, no support) It was a little tight going on, but this was no big deal and due entirely to hangies in the print. A little extra force was needed the first time, then it fit perfectly from there on out.
Couldn't be happier!