What size nut on threaded rod holding 12V cable in fuse box?

I want to wire a pigtail onto the threaded rod (or threaded part of a bolt) in the fuse box under the hood to which the heavy 12V cable is attached, so a charger can be attached to it from outside the car. This rod is under the little red cover, next to the jump point. Purchased M5 (33712-C) and M6 (33713-D) flange nuts from ACE hardware. The former was too small (nut spun about 1/4 turn and stuck), the latter too large (nut was super loose). These came out of parts bins, and hardware store customers tend to randomize those, so they may not have been the advertised sizes. By eye the M6 nut looked pretty similar in size to the one on the car, but as I said, in terms of threads the diameter was too large. Unfortunately part numbers from the bins cannot be found on the ACE website.

I don't want to mess with the nut which is currently holding down that big cable. It is presumably torqued down appropriately and I don't have a low value torque wrench. The idea was to put a wire with a loop spade on the rod, above that nut, and then tighten a 2nd nut down on top of that.

Anybody know what the thread spec is for that rod, so that I can find a nut which will fit?

Thanks.

 

Apparently that threaded rod is part of the fusible link - and it has a varying diameter. At least that is the case if I'm reading this post correctly:

Battery (post-screw) loose under fuse box. How to fix? | PriusChat

I would need to use a magnifying glass to see that clearly, looking down from the top. Anyway, if that is the issue then the M6 nut will probably fit once it is worked down past the narrow diameter tip section.