Update
Well here's my second attempt and another jig.
I'm a little happier with the fit of the pen which I tried to get as straight and slim as I could but not at all happy with the finish which again is just sanded to 400g and ancient friction polish and paste wax.
The photo is poor (must get that sorted) and doesn't show that the pen has a reasonable shine though I rushed it so time to do some research and practice on scrap to see what I can do.
The jig is cribbed of the web somewhere and I already had an old toggle C clamp with 40mm of travel. The plastic pressure pads are the feet off an old bath just turned down a little. Adjustable bolt at the end is one of the bath threaded bars and just a simple rough turned knob. I originally tapped the aluminium angle but it seemed a faff so the bar is now a slide fit with nut / wing nuts to tighten. I needed to recess the angle a little to line up the pads.
I haven't tried it out yet as I'd already assembled the pen but it should work.:smile-new:
Bob