Hey Gxiraudon!
I edited your script slightly to make use of the three hand objects Kenney now has. If you want to update your script with it, feel free, no attribution needed:
randomHand = { "Mech/hand", "Mech/hand_closed", "Mech/hand_point",}
Hand = randomHand[math.random(#randomHand)];
-- generate hands. there are no available hands so these will be simple hexagons
forge.build(Hand,{-1.01*shoulder_spacing,1.45,0},{90,0,90},{-.8,.8,.8})
forge.build(Hand,{1.01*shoulder_spacing,1.45,0},{90,0,-90},{.8,.8,.8})