Make a V shaped guide for the table router large enough to accept your sign. 2X2s would be good. You'll likely need to cut the ends of the legs to set against the fence at the angle you'll need for the V to fit your circle. Center the bit between the legs of the V, and far enough away from the fence to give you the half inch you want.
Then hold the sign in place against the guide and lower it onto the spinning bit and turn it against the rotation of the bit.
Hope that made sense.
'The true soldier fights not because he hates what is in front of him, but because he loves what is behind him.' G. K. Chesterton
Please edit your profile with a name and location so we can better assist you and make for a friendlier forum.