Thorn in the flesh/side in the OT was referencing enemies of Israel left in the land. People.
Judges 2:3
Wherefore I also said, I will not drive them out from before you; but they shall be [as thorns] in your sides, and their gods shall be a snare unto you
Joshua 23:13
Know for a certainty that the LORD your God will no more drive out [any of] these nations from before you; but they shall be snares and traps unto you, and scourges in your sides,,,,
Numbers 33:55
But if ye will not drive out the inhabitants of the land from before you; then it shall come to pass, that those which ye let remain of them [shall be] ... thorns in your sides, and shall vex you in the land wherein ye dwell.
Asking "where else in the bible is the phrase "Thorn in the Flesh/side" used and what did it mean.
Also asking the question "was it a particularly famous passage that most people would have heard of before?"
And then you must conclude that most of his readers who were familiar with these famous verses and how would have heard endless lessons on the mistake about leaving these people in the land, would think Paul was referring to people or a person.
And that is the best interpretation in my opinion due to the hermeneutical rules I just presented.