Clause 21 of the policy requires the assured to maintain with the Maritime Administrator a collateral deposit fund or a surety bond, to secure the payment of the premiums, in an amount which shall at all times exceed the unpaid premiums on all risks which have attached under the policy. The minimum amount of the fund or of the surety bond shall be $1,000. Clause 21 also provides that, within seven (7) days from the time knowledge comes to the assured that the amount of the deposit or the surety bond is insufficient to meet the requirements of Clause 21, the assured shall deposit additional collateral or increase the surety bond in an amount not less than double the amount of such insufficiency, and for a sum which shall be a multiple of $500. If the assured fails to increase the deposit or the surety bond within the seven (7) day period, the policy automatically becomes void at the end of the seven (7) day period except as to risks which have attached prior to that date. The procedure for establishing a collateral deposit fund is prescribed in § 308.509, and the procedure for posting and maintaining a surety bond is prescribed in § 308.510. An application for the issuance of an open cargo policy shall be ineffective unless a collateral deposit fund is established and maintained, or a surety bond is posted and maintained, in accordance with the provisions of this section and § 308.510.