Washer Bottle/ Oil Seperator Bottle

The problem i am trying to work out with this is not how to make the moulds but how to make the top section. The bit that the cap scews onto.

The only way i can see is to buy an alloy screw fitting and bond that into the part.

Any other ideas on this?

Bonded alloy fitting is probably easiest…very often you can get alloy fittings intended for welding in, which can be easily modified to bond in…

That was my thought, i just wanted to confirm thats all.

Thanks