The stainless steel ties from Panduit are the best to attach your witness plaque when nails aren't an option. Available in one size: 33" (838mm) by 0.31" (7.9mm)
When a natural support is not available you can attach your plaques to this 1,80m galvanized "U" shaped beam. Pre drilled holes to match most witness plaques. Also available: bolts and screws.