I took a piece of hard nylon (plastic) wear material and turned a couple of nipples. Left them about 1/32" longer than standard. Found them to be perfect, Only damage is eventual flaring of plastic, but last for many many 'shots' before that happens. Doesn't need to be drilled (obviously) and the shoulder above the thread should be left as large in diameter as possible. I used a lathe then die cut the thread but could be done without the lathe using a drill or by hand filing. Nylon is very resilient but not hard to cut. I used offcuts and worn pieces from a food factory from a factory (cause I m cheap) but it can be baught new from engineering shops, probably in diameters close to correct for the job.