[MrForbes]

I have a lathe, I don't see what the problem is. Cut some threads in a piece of metal to fit the part. I guess if you don't have a lathe, then you see the problem differently than I do.

If the 3/8 NPT tap happens to have the same thread pitch, and the conical tap happens to have the same diameter as the suction cup at some point along it's length, you might consider making a thin nut from 1/4" aluminum stock. The threads will not be a perfect fit, but they could be close enough.