If they have threaded extension on the back side, I would call them shoulder nuts.
They ma have knurled edges. That makes them knurled shoulder nuts ( may be called bushings.
The length of the bushing is critical. It would be slightly longer or as long as the thickness of the plastic. The shoulder is there to prevent over tightening of the bolt that could cause the plastic to crack.
I did some quick searches and found that looking for stainless knurled shoulder nuts gave some results but without the actual dimensions, I can't sy if the ones I saw would work ( the ones I saw were metric .