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If you have already planned to replace it then any carbon steel bolt would work. Not sure how it is attached on the underside. Field stripping the Fuse box into it's pieces should tell you.
That or replace the nut with a thinner one. A nut only needs about 1 1/2 threads to develop full strength for the joint. The thinner ones are called Jam Nuts. You can get them everywhere like even Home Depot. Google it and you will see. Mr. Sam, PE