Ed,
I havene't had a problem with vibration either put just wondering if the polymer would make things even better. Like I said, the other article I have says the Gum of Aerobic helps and people like Tywmann used it.
I think your idea would work fine, just wondering about the added complexity of the setup vs finding a simple method to make large diameter collimated light.
Here's an idea, what if you placed in the water a small flat of know quality as an internal standard along with the flat being tested. If you photograph it and the flat under test at the same time, you should be able to back out the error introduced by the thickness of the water and angle of the light by looking at how much error was introduced into the fringes on the good flat, then apply that correction to the one under test ?

- Dave