I started with a simple 3rd party NES controller for this mod.
https://www.stoneagegamer.com/nes-controller-tomee.html
I actually took 2 attempts at this, after frying the first controller.

The following are photos of the process of converting this, into an arcade style controller.

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Prepping the board.



After dismantling the controller, I noticed that the board had all it's contact points intersecting.
THIS could pose a problem with making things work as planned.
So, I scared them all down the middle with a knife sharpener.

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Soldering wires. (Not the best job in the world)



The soldering has been done!
It's as ugly as sin now, but by the time I'm done, you will never see this part of it.

Tested by jumping to a microswitch, all button inputs function perfectly!
...Also, for some odd reason I was able to use my body for ground, which made for the most awkward way I could ever play 3-D battles of World Runner, by holding the ground wire in my left hand, and grabbing the control wires with my right.... Did not get far BTW.
I do NOT suggest trying this.

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Finished product, and inner workings.


The inside of this thing.

The ground line is actually just jumped off of one part of the circuit board, and looped through all the other controls, those are the blue connectors, the red ones relate to the actual data being triggered.


The actual controls.

I know that I am slightly off center with the joystick, but everything functions perfectly.

Final verdict:
I should say that I am happy with how this came out.
However, if anything: it seems to make the games harder.

Awesome!! I'll show this to my son!