I'm going to jump ahead of myself and show four pictures. The first two reveal "before" my efforts and the second set of photos show the "after' results of my efforts. It's not nearly a perfect replication of the inlet but in my opinion after all the work involving trial and errors I think represents the best compromise and certainly is less conspicuous than its original shape.
As mentioned in my previous posts, among many limitations, I do not have access to a large heat source that will not jeopardize safety. The only alternate heat source was a hair dryer. I tried to manipulate the plastic part to make it more "square" in representing the inlet of the full sized Mustang, but I lacked the proper tools and did not have anyone to help me which would have made a real difference. If you can imagine holding a hair dryer with one hand and trying to shape the plastic with the other without having anyone else assisting my efforts, was a challenge.
I was granted access to the gardener's shed on the condition that I do not allow other people inside. His job was "on the line" just by giving me the keys. The gardener's shed had already been broken into several times in the past. Thus, my having a "third hand" was not to be.
I had envisioned several options of which one after the other for various reasons did not work out. I even thought of using some squared hardwood pieces that were slightly rounded at the corners, placed inside the plastic inlet to help give it more of a rectangular shape during the heat treatment. But the manipulations of the plastic inlet with the wooden pieces proved problematic with the plastic melting and the wooden pieces moving around within the inlet. All of this being done with just one hand.
I tried affixing the hair dryer to a vice and giving myself two hands to work with. Unfortunately, the vice in the gardener's shed was mounted such that it faces away from the table's edge.
I trust that this "compromise" will pass muster. Your opinions will be greatly appreciated.