There is a very good reason why you wouldn't really see that out there, it's a super niche case. There is this kind of
garbage out there, which just looks as craptastic as it can.
I don't think there will be much of a well built / designed market for that type of thing, as most installations won't use something as constrained as a hardware formfactor to build something as unforgiving as a speaker platform. Most DIN installations are in cars, so most speakers would be otherwise installed in the vehicle. Most desktop installations would have well built speakers that won't fit a 2.5 x 7.5 in rectangle, as anyone that is that serious in implementing that wouldn't make such compromises on the speaker. I have seen some very slim rectangular speakers that could probably be inlaid in blank plate, and had a grille mounted over it, but I think it's more makerspace material than readily available purpose built stuff.
I fear that you are either going to have to buy absolute garbage from the far-east scumbags, or build something quality yourself. The only advantage I can see from that
scheiße I linked earlier would be the additional features that come with it. It's more likely a Picasso, good from far - far from good.
Considering it's just a speaker mount, you could likely make one if you wanted.
