I can't recall who it was asking about rubber cap for alt stud, but I came across some that might work. I'm installing a new aux battery and it has 5/16" NC bolts as the pos and neg studs. I found that Blue Seas "stud insulator CableCaps", PN 4010, fit both the battery studs and the 4 AWG cable I'm using.

They seem well made, come in black and in red. Not sure if they are rubber but they feel that way.

Alistair