Kudos for thinking outside the box. Problem #1, the bills were still bundled in order, if one was to trim them they's be separated.. too hard to trim bundled. Problem #2, this pic of Cooper TBAR Bills shows a portion of the "$20" circle on the back of all of them (4 in pic)... a trimmer would not leave those, they would trim well inside of the design.
Thanks for the feedback, Flyjack. If I sound combative in my replies, it is not intended. I like you and your out of the box thinking as well. To address your first issue, I would normally agree with you about trimming the bills one by one. But there were 10,000 of them and a ticking clock, and he reportedly had a pocket knife and not scissors. He had cut apart a parachute just for the cords, so I don't think he'd have had a problem MacGuyvering this, if he was familiar with raising notes.
As to your second point, I have never found one instance of a raised note being produced by joining a trimmed $10 or $20 with a trimmed $1, and I've been researching this for quite a while. Universally, they paste the higher value over the top of the lower value, so only one side of the $20 would have been needed. Your observation - a keen one - only points out to me what I already suspect: that he trimmed for the face-side artwork as can be seen in every one of the example photos I posted above.
Based on your feedback, I will amend my language to say that no face-side corner artwork of the Cooper cash has ever been found or photographed. Excellent post. Thank you.