My experience with CAD is limited so using emachineshop program. I have been thinking about new lathe owners. Below are three views of a simple stop. I hope it helps. Use whatever material is available to you. What is not showing is a threaded connecting rod. Either use all-thread or thread a rod to suit. The part is thread through its length. I designed it to be about 1 inch long. The small diameter is a slip fit of the spindle inside diameter. The larger diameter is the outside...
Lathe Spindle Part Stop
Lathe Spindle Part Stop