To Bob - thanks for providing the info and pictures. Chris Olsen does outstanding work. It's hard to find sources which can do this level of work and for it to be factory correct. Tip my hat to Chris.
One area I'm still curious about is if there's a way to come up with rubber block-type terminals which can be applied to harnesses already installed in the car. For instance, let's say someone added a non-factory connector (which is necessary), or a repair was done and non-standard terminals were used. In this case, the incorrect terminals/connectors need to be replaced with factory correct ones. However, removing the entire harness isn't practical.
The SpliSeal approach is pretty interesting because you simply buy the mold, then lay your wire down before closing the mold and injecting hot sealant with a glue gun. This enables you to make rubber blocks around ring, spade and other terminals, barrels, etc. However, the SpliSeal mold has a logo embedded into it which we don't want. So it got me wondering about alternatives ....
One possibility is Versimold.
https://www.versimold.com/ Basically this stuff is soft rubber which you mold into the shape you want before heating it cure (250 degrees). Once cured, it becomes firm hardened rubber. Heating can be done with a heat gun, hair dryer or by placing in boiling water. Challenge here is that molding these terminals/connectors by hand isn't practical. They'll never come out perfect. What's needed is a mold of the rubber block connector/terminal you're trying to re-create the shape.
There are ways to make molds with Polyurethane which is really quite fascinating. Basically you can make just about anything. Take a look at the videos below (I'm in no way associated with BJB, they just have good comprehensive videos which explain):
Still, the problem here is you need a good vacuum system (28-29Hg pull/ 5-6cfm flow) to remove the air bubbles from the material. But understandably that cost less than the injection molding machine Midlife mentioned. Still, doesn't look practical for a few home uses.
That led me to look at other ways to make a mold. One option is air-dry clay. Clay can be formed by hand, molded, shaped, imprinted or what not, then it air dries to hardness. Seems like a good solution. So I'm thinking of experimenting with Versimold and air-dry clay.
What I would do is form a block of clay, set the terminal (wire and all) on top and press halfway in. Then I'll then flatten the surface around the edges. Idea will be to make the first half of the mold. After removing the terminal from the clay, I'll inspect for imperfections. I'll have to be sure the mold takes into account where the ring or spade terminal, if applicable, will protrude from the rubber block.
Once the first half of the mold is dry, I'll make the second half starting with setting the the terminal back into the mold and adding more clay on top. Then I'll shape into a block to match the bottom half. After removing the top half carefully, again I'll inspect for imperfections and account for the terminal end.
After the two molds are dry, I can shape some Versimold around the ring/spade/whatever terminal before setting into one half of the mold. I would need to be sure there is enough Versimold to fill the mold completely. Then I'd put the second half of the mold on top and press. Next I'd take the top half off and remove any excess material. If all looks well, I'll remove the terminal from the bottom mold and then heat it with a hair dryer or heat gun.
If I've done everything carefully, including covering/protecting the car and all surfaces, I will have been able to recreate the molded terminals right on the car, without removing the harness.
I have the Versimold, next I'm going to get the air dry clay (or whatever is best), then experiment on an old harness. I'll let everyone know how it works out.