Old OEM antenna on my GF's MX-5 NC started disintegrating, so I bought cheap "antenna" from eBay to replace it: https://www.ebay.com/itm/Universal-3-5CM-Car-Carbon-Fiber-Screw-Aluminum-Short-Stubby-Antenna/112790542748

But I did't bother reading the specifications and ended up with non fitting antenna. So I designed this adapter.

Beware! This is not proper antenna and your radio won't play any FM/AM station with it. I don't care as my GF uses Bluetooth, but somebody could be disappointed.

How It was designed in Fusion 360

I created sketch which was then revolved 360° to make the final shape.