I'm curious why you dislike using more a conductive lug/nut?
Good torque is important when pressing the busbar to the plugs.
You cannot do that sufficiently with brass bolts.
I tear brass bolts/screws right off myself.
Therefore, minimum 304 stainless steel with a similar nut, washer and locking washer.
The washer and locking washer can be copper if you can get them otherwize stainless et. al. is quite okay as they are not going between the busbar and the lugs, only on top.
What is conducting is the busbar and the cable lug to each other.
The stainless is only there to press it together and keep it locked.