I have two with C channel going down into 4 stake pockets.  One bolt in each pocket and it will NOT slid when the liquid hits the front or back.  (maybe the tank will blow!!! lol )

Unbolt, two forklifts and drive out. got an old running gear that will hold two of my setups on when not in use.

Also I line the whole flatbed with liner (we sell it and install containment liners) from the front to the bottom of the beavertail.  Any spills run off the side and not down on the axles and frame.

Also, all valves are threaded into the tank here.  I see you have a hose that is direct into the tank,  one thing I would have you change if you could.