Using the scaling principles mentioned in the post, I've made a collar for an Olympic bar (50 mm diameter). I basically doubled the size and removed some material in the middle.
Unfortunately I don't have an Olympic bar in my possession to test out the design, I'd have to wait till I go to the gym, although I'm confident it would work. That being said if any one has a Olympic bar and would like to test out the design on that, I'd be grateful, and interested to be informed of the results. If you'd like to try, the model is linked below, thanks.