I bet you get a million suggestions...I have two: original gorilla glue and gorilla glue THICK CA. Original GG is not the best choice here. It expands. But if you use a toothpick and just apply a tiny bit, then lay down some wax paper and a small weight on it, you will be good. You may have to pick some expanded stuff off the next day--this tells you that you used too much. GG THICK CA is a cyanoacrylate (sp?) glue with a thick base. Again, toothpicks and tiny amounts. Let dry overnight. Then hope you never need to get those signs off because they will NEVER come off.
Good luck.
Don Merz