Simon,
Thanks for the encouragement about entering the 4th year of build. I'm in the third year of building a BR17, (commenced boxing day, 2010). I'm a week or two away from starting the paint job on the exterior, so I am facing the same problem.
The instructions for the BR suggest using a joined-up rubbing strake - but I rather think I want this stuck to on the boat before I put any paint on. I was considering using a manually levelled spirit level which I already have - about £30 as I recall.
I like the plastic tube idea - probably just the thing for levelling the boat in the first place too. I might give that a try.