[MBAtarga]

The pictures don't help me understand the orientation of the parts and how they would need to fit. I understand you are saying the square tube section needs to fit within the receptacle with the ribs. Could you insert/mount a rod in the square tube that would readily fit within the ribbed part?

Edit - you added more pics before I typed my response. I didn't realize receptacle is plastic (I thought it was cast aluminum) AND that it actually was flexible and can be opened up. Take the part to someone with a bandsaw and those extra ribs could be cut off in two minutes.