I found out about polymer clay in this post (the thanks button is disabled for some reason...).
And I used the resistor values listed here, as some other threads showed they also worked with the Nexus S: Fun with resistors (home/car dock mode + more)
I ordered a Sparkfun microUSB breakout board, bought some resistors at a local electronics shop and four blocks of black Fimo clay at a nearby arts shop. I had the other supplies on hand:
-Soldering iron, solder, etc.
-Shrink wrap in various sizes.
-Narrow and slightly rounded file.
-Box cutter or exacto knife.
I made the cable first (and did not take pictures at that time) and ensured that part worked before tackling the clay portion of the project.
I soldered one end of the resistor on the breakout board and the other directly on a stripped portion of the ground cable. The data wires remain functional as I am using a wall charger that properly shorts the D+ and D- wires to enable AC mode. I secured everything to the board with hot glue and finished off with shrink wrap.
This was my first time working with Fimo and it went well. I wrapped my phone in cellophane for a test fit along with the modified cable. The cable however isn't rated for the baking temperature so this was only temporary.
After baking, it turned out that I had misplaced the portion meant to hold the cable, so I snapped it off and made another with some left-over clay. The cured material can handle rebaking with no issues.
A bit of rework on the attachment point with a knife, test fit again, glue the board and file a notch for the buttons.
The phone slides in and out smoothly. I'm quite satisfied with the results.
Update: dock still works great. I've made one mod though: I took an old mouse pad, tore off the cloth on the top, cut a shape matching the dock's bottom, and hot glued the two together. This anti-skid pad means I can turn off the alarm or answer a call (which the phone answers in speakerphone mode right away) without inadvertently moving my phone on the nightstand.