When doing A lot at one time (several molds), put a sacrifical fence on your bench. It can be as long as you want. This fence is stationary. Utilize a hydraulic ram or air ram and mount another fence of equal size on the ram. You could do it with a hydraulic arm pump style but I would much prefer one that runs from a hydraulic pump with lever to activate and release ram. I've seen one that had a foot activation.

Set all the molds on the stationary fence, push the foot pedel, the ram slids forward clamping your molds. No screwing around with "screwing"!
Obviously, this wouldn't be a one or two mold deal unless you are like me and hate wasting time or are always trying to reinvent the wheel!