When flat was new I too did not want to go through bottom of boat to hold down brackets. Made wooden cradle's that fit tanks. 4. one on each end of each tank. glassed to the floor. Drilled and put eyelets in them and used screen door spring encased in clear hose. the springs attach to the eyelets. they have not moved in 30 years. and no holes in floor of boat. should I need to remove a tank, remove the hose to gas caps in top, remove springs on one side and lift them out to clean or sevice.
Maybe can get a pic....... works slick...........this was a Jerry Sides suggestion. (boat builder).