The treasurer shall on his appointment execute and file with the secretary an official bond in such amount as may be fixed by the district board of directors, which shall not be less than five thousand dollars ($5,000); and shall thereafter from time to time execute and file such further bonds as may be required by the board in amounts fixed by it, which amounts shall be of at least fifty percent (50%) of the maximum probable amount of money in the treasurer’s hands at any one (1) time. All such official bonds shall be executed by a lawfully qualified surety company.