Thanks for the replies. I agree, adding a tail will improve stability. Alan had a Vetter tail on the bike and he stated he had no problems with stability.
From the Vetter site: 2011 Vetter Streamliner-Chap 50
Alan stated: "Passing semi-trucks usually caused a minor wiggle. I think that I have been lucky to have a stable motorcycle to start with. Very strong continuous cross wind did push me around but not something that I could not handle. Just lean into the wind and continue at speed."
I will not try to reinvent the wheel and will be going for something similar to Alan's bike, shown here, with the windscreen and wheel fairing:
