Project Failure - 3D printed Speaker Housing

First a little background information about the project. I am replacing the 2"x4" speakers located in the headrest of a 91 Mazda Eunos Roadster, also known als Miata or later models go by MX-5. The original speakers are bad, very bad. And they tend to don't live long. I already replaced them once and after reading around the forums everybody recommends buying some normal 3.5" speakers and cut some of the foam out to make it fit.

I figured, why not 3D print a speaker housing and use the existing mounting holes that are already in the headrest.  As you can see below, I have drawn up a CAD model and 3D printed it. All looking good so far. But when testing it in the car I discovered that hidden under the foam the is one big metal plate with just the 2"x4" hole for the original speakers cut out. Because the cutout isn't big enough i don't have enough depth to nicely fit the new speakers.

I will have to sleep on it before I decide my next course of action.