I don't really want to use granite as a base, as it would be even more of a pain to move (I am in rented accomodation so am going to have to move the garage in 18months or so), and storage is a bit of an issue.

I do like the idea of a diy level though, I guess the important thing is to make the curvature constant and very small near the point the bubble sits. So the mounting wants to be as symmetrical as possible. I will see what I can knock up. I guess perspex tube would work too, though it wouldn't be quite as stiff so the curvature might be higher over the lump. I will have a go and see how it goes.

Was the glycol, to increase the viscosity of the water? or do something to the surface tension?