Extend the clamping capacity of your irwin clamps by coupling two together.

Remove the head of the clamps and use this print to couple the bars.

Rotate to print both parts flat side down.

Embed M6 nut into the hex depression with either heat fit or superglue. Use M6 bolt to connect the pieces.

I glued both halves together just to make things less fiddly when attaching to the clamps. but it's not strictly necessary.

My initial test print had the M6 hex too tight but I just heated the nut and pushed it in whilst hot. I did loosen the tolerances in the model after this but have also included the 3d fusion file in case people want to tweak anything.