OK... here goes a dumb question
If I just solder two 50 ohm BNCs back to back, there will be something like 25mm of PTFE-covered centre conductor which will have no shield. There will also be about 5mm of centre conductor which will be completely bare - where I will solder the voltage limiter.
How bad will this be at ~300MHz?
I could create a shield for the PTFE section, either with a machined brass tube or by painting it with silver paint, if necessary.
I will probably use a little metal box for the whole thing, just to make it neat.