Thanks all for the supportive feedback!

Coming back to my initial question and the different statements given, I summarize that even if a suitable solution fo fix a square section as rearbeam to the hulls is found, a platform using rectangle 'tube' 100 x 120 x 2 mm wall as rearbeam will be LESS STIFF than the same platform using round tube 100 dia x 2 mm wall as rearbeam?

I thought that the 120 mm height of the sqaure section aginst the 100 mm height of the round tube would REDUCE bending of that beam and therefore would reduce the flexing of the platform. But following Darryl, this assumption is wrong?