If it's not as long as you'd like why not extend it by welding an extra bit on from one of the other pipes? Properly prepared egdes at 10mm thick would be a doddle to weld with a stick welder.
I've done two round FBs now, and my 3rd build is a square. I prefer square as it's way easier to work with, make vents, hang doors... but there's something asthetically pleasing about a round FB. Personal choice, and it's all yours to do as you wish.
![Happy :)](./images/smilies/1.gif)