If the board of directors of any improvement district deems best, instead of issuing bonds, it may direct that warrants shall be issued or drawn on such district, or on the county trustee, by the county mayor, to be paid out of the funds of such district only, and at such times as the assessments may be due, or as may be deemed best, the time of the maturity of assessments as fixed being considered, such warrants to be issued or drawn for all lawful demands on such district, and to bear interest at not more than six percent (6%) per annum.