I needed to extend an RG 58 coax with LM400 low loss coax. It's in a marine environment so it needs to be well protected from the elements.

This is what I came up with. I used PLA but I will wrap it in self amalgamating tape to slow down UV degradation.

https://www.tinkercad.com/things/8SQsNwk9NGb-rg58-lm400-coax-junction

20/10/2023 update: after over three years in the field, it's still solid.