I started to edit the previous post, but too much time passed, so now am adding fresh addendum to post the photos I sent to Bill.
I sent them to Bill because I knew he had a saw like mine, and wanted the benefit of how he solved the issue. Turns out, he solved it by switching tables. But he had lots of great suggestions, as he always does for any and everyone on this forum, about how to make a template.
First, I already have a negative template... or an inverse template... which is the table itself. This is the shape I needed to conform locally sourced ZCI blanks to. (Local source = Rockler store nearby)
I don't know how to use a trim router bit to transfer the interior pattern that is BELOW the perimeter seat for the throat plate. While I realize that the perimeter seat is adequate to support a ZCI, since it is all that supports the original flat metal plate, I still had to clearance the Rockler ZCI blank to fit within the protrusions below the perimeter seat due to the thickness of the Rockler blank.
Ideally, I wanted an interference fit within the protrusions below deck, since the phenolic plastic ZCI weighs a lot less than the steel plate that it replaces, and the lighter the throat plate, the more likely the throat plate can be unseated. Since the phenolic ZCI was much thicker, I wanted to take advantage of that thickness and shape the bottom side so that it would lock in edge to edge with the below deck formation
, which would contribute to the throat plate staying put, since there are no retention screws or clips that would otherwise hold it in place, unlike the original metal plate that is retained by two spring clips, as well as it's heavier metal mass.
But alas, my inexperience with milling and replicating 3d patterns such as this has been a hurdle that I have not yet been able to overcome, so I welcome any advice about how to replicate that shape.
One idea Bill mentioned was to make a positive template out of bondo, by first masking off the surrounding area, and then applying a release agent to all the undulations that I want to replicate, and a temporary bottom to hold whatever epoxy I make the positive pattern out of in place until it cures. Then, hopefully the release agent (silicone, saran wrap, something) will permit a clean removal of the "false" positive when cured, which I would then do what with?
Place upside down on a work surface, clamp in place... then? I just don't know how I would transfer that pattern to a piece of hardwood or another piece of phenolic. It seems like sandwiching them together for the trim router to do it's job with the bearing riding on the pattern and the cutter riding on the material to be cut wouldn't work, because the edge is layered and complex, and the rise of each rabbit layer is as important as the run. So I can't get my mind around how to make it happen... but again, I welcome any and all instruction.
Left to my own devices, the only way I could come up with was free handing with an end cutter (for making dados) on my only router, which has a base way too big for this kind of work. The base didn't have much to ride on when working the edges at only a limited depth, and half the base of the router was unsupported, leading to my fighting to keep the larger router level, while still free handing the cut. The results were not pretty.
I would have liked to have done better. Much better. The insert now fits OK, without riding high on the below deck protrusions before making full contact with the perimeter seat, but I did not achieve the below deck interference fit with those protrusions that I originally wanted. My router was too big (1613EVS) for my inexperienced hands to control entirely freehand with no pilot bearing (because there was no edge that the pilot bearing could act on, as the cuts were only partial depth.
As it sits now:
Since I need to make two more of these (one to clear all bevels up to 45 degrees, and another for dado work), it would be a relief if I could find an easier solution or make better renditions the second and third time around.