So, you need to find a speaker that has its mounting holes that fit those four holes in the cover
I'm not sure that is the case - in my first pic, you can see the holes in those corners are at an angle, to match the cover. I don't know of any speaker frame that would accommodate that. If anything, it seems like those two small holes
The little I can see of your car's hood from your first picture of the speaker opening does not suggest 65 or 66 Dodge Plymouth or Phoenix to me, but I could be wrong.
That's because while the exterior matched the same year Fury for most of its production, the interior from 65-68 stayed the same 65 Monaco dash, just mirrored as RHD. I wonder if Chrysler ever made RHD Monacos, because they went to the trouble of mirroring the dash, but the wipers are old-school clapper style, rather than a parallel action just mirrored across for RHD. Maybe they decided it wasn't worth making a whole bunch of extra wiper parts for such a small market.
The pic is kind of shitty because it's the dead of night here so I'm just relying on the camera's flash. I measured the edges of the hole and it's exactly 4x10, but that's just the hole, not the surrounding flange.

Also I just realised that you can see the speaker grille where I left it as well lol.
@Big_John I was thinking of doing something like that but with some sheet steel, as the Custom Autosound speaker has its own cover that seems to be about 1/4 inch thick and first impressions are that it would be pressed against the speaker cover. Wonder if that alone would hold it in place if I were to bend the steel flanges on the speaker itself flat...