A coupler to connect two Harbor Freight Ratcheting Bar Clamps together. Should fit any size except the 6".

The original  thumbscrews that hold the stationary stop are long enough to pin the coupler in place, but not long enough to screw into a nut on the opposite side, so you may want to get a longer bolt if you want to be more secure.

I printed in PLA, but another material might be a better choice depending on the environment you use/store these in.

Also, print laying down as oriented in the profile for better strength.