I had same issue with my Sony headunit and aftermarket fascia. The Sony cage would not fit in the 'internal' part of the aftermarket cage. Only way to make it for would be hacking the plastic as your installer has done. As you say, what he should have done is use an aftermarket cage and not the one supplied with headunit. The after makert one will also allow you to move the headunit in or out to make it fit flush.