8MM Linear rod clamp

thingiverse

I suggest using some blue tack or an equivalent adhesive to center the object firmly in place. This ensures a tight grip, although it's not strictly necessary - it can be very helpful nonetheless. By doing so, you'll find it easier to clamp a round rod into a vice designed for rectangular objects. I've personally found this trick useful on several occasions and it's become one of my go-to techniques when working with such vices.