If I remember right, the 1/4" Alumalite is rated for over 100mph winds. (Don't think the one sided Econolite has this rating.)
I have made several signs using this material with no supports top or bottom.
I put about 2" deep slots in the posts to slip the ends of the sign into. Then put about three lag bolts through the
post, slot and sign to secure it on each end.
Makes it difficult to remove the sign though, unless you cut the slots all the way to the top of the post to slide the sign
out.