A district is governed by a board of at least 10 but not more than 13 directors, appointed or serving as follows:
(1) two directors for each security zone appointed by the commissioners court of the county and nominated as provided by Section 68.152;
(2) one director appointed for the district at large by the commissioners court of the county under Section 68.153;
(3) one director appointed under Section 68.154; and
(4) any director serving under Section 68.155.