You can get the "Roll Control" racks cheaply. Look on ebay etc. I am going to invest in some soon for my Whaler. I imagine what your calling a "fishing boat" is some sort of offshore center console type boat. These are also excellent family and ski and water sports and all that type boats and they are roomy as well. If it is a "bay boat" perhaps they are not so good for offshore. They have less draft, less freeboard and are meant for exploring shallow water.
In any case, thus far I have been laying the tanks in the forward deck of my 19 foot offshore center console Boston Whaler. They can bounce around up there when I go airborne so I am all for securing stuff.
Have fun, take only memories, leave only bubbles!!
N