I just found an aluminum sign which is 18x12 that I think will work. Will drill it and mount with ss pan head bolts will let you know how it works out
If it's flat stock, it won't have much strength, which is why I suggested angle bar. The angle bar will give you a lot of support, whereas the flat sign stock will just flex when it has weight on it.
Here's an example from Home Depot.
http://www.homedepot.com/p/Everbilt-3-4-in-x-36-in-Aluminum-Angle-Bar-with-1-16-in-Thick-801377/204273984I would suggest something a bit thicker, like maybe 3/16", but the 1/8 might work fine