I think there is a simpler way. Why not clamp the plates together? Either overlap them and apply a clamp or butt them end to end while using another pair of plates to "sandwich" the joint. Use clamping to compress the sandwich. I expect that a clamp will be enough to hold the 20KN you describe. If you need more force, increase the number of clamping points being used. This solution does not require you to alter the 6x60 plates at all.