Subject to the approval of the workers’ compensation commissioner, any employer or group of employers may enter into or continue an agreement with the workers of the employer or group of employers to provide a scheme of compensation, benefit, or insurance in lieu of compensation and insurance; but such scheme shall in no instance provide less than the benefits provided and secured, nor vary the period of compensation provided for disability or for death, or the provisions of law with respect to periodic payments, or the percentage that such payments shall bear to weekly wages, except that the sums required may be increased; and the approval of the workers’ compensation commissioner shall be granted, if the scheme provides for contribution by workers, only when it confers benefits, in addition to those required by law, commensurate with such contributions.