If your cabinet is of the style where there are little strips of wood the top insert sits on ... then you can go to a plastic company and have them cut your insert. You will need to provide the pattern
which would include the outer edges and the sewing machine area.
If you have trouble you can PM me as we have an excellent plastic company here that has cut out numerous templates and a few extention table tops for me. They also cut out the insert that I got with my cabinet that didn't fit my newer machine. Well worth the price.
ali