Spacer between sleeper and post stirrup

Hi guys,
I am putting a pine sleeper up against the stirrups that hold the deck posts. The stirrups are galvanized and into concrete and have not rusted.
I don't want then to rust where the sleeper goes up against them.

My concern is that the metals in the CCA treatment of the sleepers do react with the galv, and cause it to rust. I found this out from an excellent post in this forum.

To avoid rust I have painted that side of the stirrup with anti-rust paint but it doesn't stick all that well to the galv. On the high bump points of the metal it rubs off fairly easily.

Also I have painted the sleepers with Bondall 4L Inground Timber Protector https://www.bunnings.com.au/bondall-...ector_p0960509

But ideally I would like to separate the sleeper from the stirrup to avoid a "moisture trap".
* thin bit of hardwood? Would I know if that was CCA treated also?
* untreated pine? Good but would rot, methinks.
* thin bit of plastic e.g. like a solid 3mm diameter straw of plastic, placed vertically?

I like the last one because plastic won't hold moisture, and if I put it vertically any damp runs away. Just provides a little gap. Remember when you would buy plastic toys that needed putting together and you twisted all the little parts out of the one-piece moulded plastic frame? I am thinking of the plastic frame stuff, it was about 2mm or 3mm piping.

Might seem unnecessary, but if the stirrups do rust its a gigantic nightmare to fix even if I am just avoiding this for the next guy.

Thanks,
Dave