I am using caps on my XT30s for ages now, and wanted some for my batteries with GNB27 plugs as well.
I was rather surprised, that I couldn't really find anything that took my fancy, so I made them myself. This plug is not that rare, is it?

Use soft filament, and set the seam to random in your slicer, to prevent splitting.
If the fit is too loose of too tight, just slightly alter the size in your slicer.