Susan,
To see what would happen, I carved an old uneven piece of 2 X 10.
It worked well for me and perhaps you could do your project this way.
You are working with quite a bit of weight and I hesitate to add weight to it,
but with some supports for the ends, you should be OK.
I screwed rails on each side of the board, but other fasteners could be used.
Just make sure that the sensor has something good to see across the ends.
And, of course, make sure that the brass roller has something good to contact.
I just used masking tape to provide something for the sensor to measure by.
Ken,
V-1, 2, & 3
When the People fear their Government, there is Tyranny.
When the Government fears the People, there is Liberty.
- Thomas Jefferson
You must be the change you wish to see in the world.
- Mohandas Gandhi