How strictly do car stereos adhere to the official ISO DIN sizing? Are any brands smaller/larger, and is there any way to find out prior to purchasing?

I have a mk 1 Ford Focus that has a screwy factory head unit, something like this shape:

I fitted a Double DIN JVC head unit (this one I think) a couple of years back along with a reversing camera, but it just barely fits top and bottom (even with some judicious hacksawing), but with open gaps either side. I tried a couple of aftermarket AliX fascia options to hide the gaps but couldn't get anything to fit right and it has been ignored since.

I'm now shopping for a head unit for another car, and it occurred to me I could shift the existing JVC head unit to that car, and buy something for the Focus that fits better. Ideally I want something a little smaller and shallower (i.e. x, y and z all smaller) than the existing sized head unit. I can then use the existing fascia I've got to fill the gaps.

Alternatively, it may be an option to put a single DIN stereo in the Focus and get much bigger fascia. I'd need something with a screen for the reversing camera though.

The car is now over ten years old and I don't want to spend more on the head unit than the car is worth (though it is potentially an option to keep the stereo for subsequent car upgrades).