Maybe a piece of foam behind the bolt head if the bracket flexes. Might be a problem if the bolt head taps against the JB when/if the assembly vibrates. The JB is evil when it's not happy.
I cut the bottom from an aluminum frying pan, as wide as the r/r, with two bends, then bolted with the two battery box bolts.
Ima thinking rubber like the battery's stock seat. This may look flexible, but it's pretty stiff and would take quite a knock to pull it off that case. That given, though, the plastic of the fuse holder cover is still going to feel every tiny bump over time. Dab of glue and a rubber o-ring or grommet might be a finishing touch for under the bolt head.