Hi Simon, it's not easy to pick up a ready-made mast suirable for junk rig, so a lot of us make our own.  Either way, you need to know suitable dimensions, which is the difficult bit!  A good source of such details is the collection of files by Arne Kverneland, available here: https://junkrigassociation.org/arne  in particular, he's got a couple of very good discussions of sizing and making masts, see Chapter 6 and 6b on that page.

Some of the calculations get a bit mathematical, and seem to suggest 18-20cm diameter, which feels a bit much to me (my 29 foot boat has a 20cm mast).  I think you could do well with a 5m length of 100mm (edit - no, that's too light.  Go with Arne's suggestion of 130-150mm) or so alloy tube, topped with 3 or so metres of solid wood.  But look at the numbers!  That might be on the light side (edit - yes it was!).  

What's your expected sail area?  Mast length above partners?

James

Hi Simon. I'm in the early stages of converting a 20' Hunter Medina in Cornwall. I have struggled to find a one piece mast. The Norse Hydro company in South Wales looks like it might be going out of business so I'd be careful about transferring any money on a long lead time order. And the Hampshire company, who's name I forget are no longer making one off dinghy masts. So I am doing a three section mast, from decreasing diameter aluminium tubes. David Tyler and I are working separately on 3D printed joining and fairing sections which i think will make the joints more acceptable. I'm going to be sewing a split junk ring, which I hope will allow for minimal mast position movement. My biggest issue is lack of funds but I'm sure there's a way of doing it at minimal cost. My second biggest issue is how to step the mast at the beginning of the season. It was fairly easy with the original mast and an A frame but dropping a tubular mast accurately into a hole is too much for my understanding. Good luck with your project, looking forward to the details.