Volvo 240 Antenna base

Was trying to replace the volvo 240 wagon atenna but the replacement is too expensive, so I am making a custom base to attach a generic antenna to. Currently reusing the outside housing and securing nut because theyre nice.

Caveat: I think volvo used some custom diameter for this because I have to use horizontal expansion to cheat a little. I use -.5mm on the joinging pipe and 0.3 on the base. Results may very so you have to fiddle around with it. the threads are definitely distanced 1mm apart from each other.

I put my caliper up to the threads (of the original base and securing nut) and they read out 12 +-.1mm