I had a similar problem installing our cooktop in an old kitchen I was given. The hole was OK front to back but too wide. I used a couple of strips of stainless steel sheet which I glued to the benchtop and then sat the cooktop on top. I made it finish flush with the front of the cooktop.
If you want to use timber, you could just make up a frame (like a picture frame) the right size for the cooktop to sit in and wide enough to cover the hole in the bench. Screw it to the bench from underneath, or just glue it with a bit of construction adhesive. I'd say any decent hardwood would do the job. Seal it with some polyurethane or maybe tung oil or similar. Or you could paint it with an enamel paint to match the bench or the cooktop.