Putting a tinny on a trailer and bouncing it down the road works the hull skin, stick a non-flexible box with welds in the middle of the hull skin and the skin flexs all the way around the box and the welds work harden and crack.

 

Putting a box in a tinny that goes on and off a trailer is a bloody dumb idea - it will fail eventually due to the mechanical properties of aluminium.  Unless, the job is done properly and then it will cost a hell of a lot more than $40.  Won't fail though...

 

Vegetable oil isn't the best to use near aluminium either.  When it ages it becomes acidic and creates acids that eat aluminium.  Or it decays and forms seperation layers and grows bug and stuffs the sounder up.  Pretty much does similar things to diesel that is allowed to get into contact with aluminium actually (have just removed two aluminium tanks off a launch that have had pinholes eaten into them...).  You should be using glycerine or antifreeze solution as the ideal.