Somebody, somewhere, would probably put that new thermostat housing in a lathe and just cut the opening bigger till the thermostat fit, or till they ruined it.
That’s what I’d do. How could you possibly ruin it?
Worse case scenario is the thing is already garbage but if you can’t walk up to a lathe, indicate the thing in and open up the hole correctly you shouldn’t be walking up to a lathe in the first place.
And that’s not directed at you personally. I used you in general terms.