5/16" is 7.9375mm - I doubt the delta will make any difference, especially since you are clamping on a thread.
I'm willing to make these for you, what quantity do you need ?
I estimate the price I'd ask around 1.5 euros per clamp - not including shipping.
Do you have any preference for color ?
For material I'd recommend PLA as ABS would be softer against the thread, but that's up to you.
Most of my technical comments should be correct, but is THIS one ?
Anyway, as a rule of thumb, always double check what people write.