If you don't need the clearance that the nylon spacer provides, I'd definitely go with a steel flat washer & lube between the bars... or even a 'wavy' spring washer (as used on air-cooled VW engines) under mild compression from the nut and bolt. Probably obvious, but the tighter fit the bolt is in the hole the better, and if you can find a good fit in a bolt without threads for the first bit under the head, that'll help. Maybe go to a single deformed-thread locking nut? You might could lose the flat washer between the nuts & bar; you only need movement between the bars. The stack would be bolt head, bar, spring washer, bar, locknut. Use some kind of paste lubricant... I'm partial to a silicone paste that my buddy / mechanic turned me on to.... 3M brand, IIRC.
Jim, "There's no right way to eat a rhesus."