Weir Boxes are the preferred method of monitoring seeps from concrete and earthern dams.  Their ability to meaure low flows and retain sediment for future investigation make them a natural choice for the application.  The small foot print and ability to have differnt inlet / outlet configurations also are advantages of weir boxes.  

For dam seepage applications, stainless steel is a common material - providing strenght, longevity, and a resistance to vandalism that is difficult to match with other materials.

In areas where the site is protected from prying eyes (and hands), the lower cost and robust nature of fiberglass can be an advantage.

Regardless of the material or configuration, all weir boxes must conform to the ISO sizing guidlines for thin plate weirs.  These guidelines describe the length, width, and heigth of the box and are based upon the maximum design flows and the style / size of weir notch.  Boxes sized smaller than the ISO guidelines will have higher approach velocities and the indicated flows will not be representative of what is actually occuring.