Yeah, trying to get this specific size while making sure it is VEX legal is going to make things WAY harder for you than they should be, at least I would think so. My suggestion would be to either use the thin spacers, since there is more widely available, or to use 4 post hexnut retainers, which can produce a very similar result to what you are trying to get in your CAD screenshot. Considering the route your design is going in, I would suggest trying the 4 post hexnut retainers out and see how you like them. Good luck out there!