What options are there for good Low DC Voltage, High Amperage Alligator Clip connections?
Honestly, 20A isn't high-ampere in car electrics. The starter has 250A. My advice would be to neither use starter crocodile clips nor the terminal blocks. They are both suitable for 300A minimum, that's 15 times overkill for your application.

Pick a connector from a car parts store which is rated 25A and you should be all right.