A simple beaker clamp, adjustable to hold a number of different sized beakers, albeit not simulateously. 

To secure, drop the circular end of the beaker clamp over the mounting stand, and self thread an M4 bolt through the hole in the back.

To secure the two arms together, run an M3 bolt through the joint holes and secure until snug with an M3 locknut

To make the clamp adjustable, run an M3 bolt through one arm hole (near the clamp end) and place a spring over the bolt. Push the rest of the bolt through the other arm hole such that the spring is secure between the arms. Thread a wing nut on the threaded side of the bolt to act as an adjuster knob.