How about using gum paste instead of cookies? What I did 4 years ago is I made cookie-colored gum paste and cut it out, after they dried completely I decorated with royal icing. The gum paste is going to be rock-hard and what I made is still here. Just some colors of icing fades faster.
Let me find whether I have the photos of it 4 years ago and I take the photo how it looks now.
I made gum paste dough with gum paste powder and followed the instruction how it should be made.
Here I found the photos