Would use a clear silcon SCS1000 by GE. This will allow the sign to be removed in the future without any harm to the drywall (Can leave a water mark on the paint thou). If that is not a issue then 3m VHB or other various double sided foam tapes will hold it forever.
There are small metal hangers that actually have two angled points to shove right into
the back of the foamboard. The work great and are very simple.