For my 2 batteries
 

I like these:
very safe

lok-knobs.com

your talking for the accessory stud.......can be easier to torque as long as it is high enough to clear the post fitting.

how about for the big starting cable that goes on the post.
Those lighter cables that go on the accessory stud don't carry enough amps for a starter.....may starve it of volts.
A bigger cable with a big lead connection on the post is needed.

definitely works for that. Plenty of clearance

5/16 is standard?

Ordered two pairs in 5/16. We shall see if I guessed right... My batteries are not easily accessible so any help will be appreciated, especially not needing a wrench. (although I think I might have already have wingnuts on them)

Wingnuts aren't legal Ross, not that I've ever heard of someone getting an electrical audit by the CG but wingnuts tend to loosen up.

Most marine battery posts are 3/8"-16 positive and 5/16"-18 negative
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