Thanks, all three of you. Joe, I bet you're right, but I'm scared to try it. It's a big sign and the material was not cheap!
Martin, I tried Rest Machining first, following the video. I'm not sure what's different, but I couldn't get it to create the component from the preview with the offsets, like in the video. Tried for quite a while before giving up on that. I will come back to it later and experiment with it more.
Highpockets, that did make sense, thank you very much. It took me some more study to understand using node edit to cut the olive branch, but I got it. After that, the rest fell into place.