After working my way through batch normalization, and giving Ice & Steam point calibration a shot, I finally bit the bullet and laid down $200 for a Reference Thermapen ( accuracy ±0.04°C) which paid for itself with a huge reduction in the amount of time it takes to do the job:

DS18B20 Calibration: We finally nailed it!

All-in the current system set me back about $300 including the T-pen, drill bits that cost more than the aluminum, and a clunky old water bath that was only $25 on eBay since its front display is off by about 5 degrees. To be honest you could skip that last bit and just use some warm water in a bucket to generate those cooling curves.