Thankfully I test fitted it before painting! It was a rubbish fit as it came out the box. What I did was trim the lip around the back with a slight bevel using a sharp craft knife, I actually removed a fair amount of plastic in the end. Just test fit, trim, fit, trim.....until it sits flush. It only needs to sit in the hole, the metal frame that holds the stereo and dookit will hold it in place.
If you will allow me a gratuitous metaphore.... when it's new it's like fitting into a 18yo girl, it takes alot of persuasion, perhaps the gentle application of force, and is a bit too tight for comfort. We don't want it to fit like a hotdog in a hallway either, be carefull not to be overzelous and pound away at it, or in time it will be bouncing around like the bearing in a rattlecan. I think you should be aiming for mid twenties fit, not overly troublesome, and snug enough to make you smile.