An employer engaged in any of the exempt occupations listed in section 72-212[, Idaho Code,] may elect coverage thereof by a declaration in writing of himself and his surety filed with the commission that the provisions of the law shall apply thereto. Unless the effective date of such coverage is otherwise fixed in such declaration, coverage shall be deemed effective as of the date of filing such election, if the employer also files simultaneously or has on file approved security under section 72-301[, Idaho Code]; otherwise, such coverage shall be deemed effective upon the filing of approved security.