I think you made a good call with the rubber backing. Those instruments covers have some flex and take a bit of beating from vibration. Seems like a fine line to walk; you don't want the bond to break, but if it's overly strong you might see new fractures at the edges of your backing material.
Al, if you hackerspace (or whatever they call the place where you did your powdercoating) has a big 3D printer, it would be way-cool to see a fresh cover made from scratch 8-)
P.S. If you can find a chemical that will melt the plastic, you might be able to carefully use it to feather the edges on the outside of the cover and hide the joints