6" bolt. three nuts 2 lock washers and 2 regular washers. Bolt through sign. Nut on the back of the sign. This will keep your sign from sliding down the bolt. Then another nut at the distance you want it to stand off. Then you can push your bolt until you hit the nut and it will be spaced correctly. Then you need to decide are they going to be floating in pre drilled holes or do you want to tighten it on the other side of the bolt.
I recently did a sign for our own use. It was a routed aluminum logo. So i didn't want any standoffs showing on the face of the sign. So I bought elevator bolts. They have a completely flat head that is about an inch around. Glued those to the back of the sign. 1 inch vinyl spacer on the bolt and stuck them into predrilled holes I made a template for. Glued them in the holes and done. Super simple, cheap, fast. I used VHB tape on the bolt heads to hold in the right position and then gorilla glued all the outside to the back of the sign.
Attached is a picture of the method you want to do. Assuming though you would also do all four corners not just the one shown.
