There will be a safety issue if you manufacture them, as they will need to be certified for pressure handling.
I could modify the existing fittings to give exactly what we need, at a minimal price per unit, but i don't want to get involved with certification, and the associated insurance issues.
The other requirement, is that the fittings are readily available from a variety of sources throughout the paintball world. This is why I've suggested using an existing product, rather than a home made alternative. The male fitting shown in my post is around £3 - £5 depending on the source, but obviously the check valve would need to be looked at.