What I have done is to make an overlay. I bought some of that transparency material used to make presentation slides (powerpoint presentations). Then, I scan the panel decal. In a photo editor like Photoshop I put white circles exactly over the instruments. I make sure the rest of the panel is a very dark black.
I then print out the panel on the transparency material, cut it out, and sandwich it with the decal. It adds a little bit of depth. Sometimes in larger scale like 1:32 I add a sheet of thin acrylic or clear polystyrene between the two for a little more depth.
I use white glue or Microglaze to glue the sandwich together.