[AdamHeard]

I usually wouldn't use a COTS turnbuckle to tension chain as it just seems big and unweildy..... But, if you got one that is about the length between the inner bolts (to the center of the robot) on one stackerbox and the inner on the other, and the eyelets were about the clearance size for a #10, you could tension both chain runs by pulling the gearboxes towards each other.

EDIT: on second thought, get a turnbuckle with plain threaded ends instead. Make a block with a threaded hole for that end on one face, and a clearance hole for a #10 on the perpendicular face (not vertical). Put that block on one of the inner #10s on each of the stackerboxes, and hook up the turnbuckle to them.