You don't need any adapter. Just remove the old stove and the new one fits in place. They should both be a standard 30". As long as there is no countertop strip at the back that is. If there is a strip of counter at the back, then you need a contractor to remove it and the stove slides right into place.

This is correct - a slide-in will fit as is (though I will need to fix that strip at the back). In my case the gap is very uneven so really want to fill in the sides for aesthetics. I did find this one site that makes filler trim (link below) but looks like a custom order. Seems like a common problem so baffled why a simple trim filler isn't easy to find.

Here is a link that might be useful: Microtrim

There are a couple of options.
If, and/or when you have a clean opening you can have the edges squared and polished, squared and a trim piece of granite added or squared and a piece of polished stainless or aluminum "T" metal added.