Learning generally boils down to "Repetition or the avoidance of pain", some people learn by doing, some by watching and some just have to pee on the electric fence.
Whether you think that you can, or that you can't, you are usually right. Henry Ford
A square firebox is always easier, in my opinion. You can build it to the exact dimensions you need and it is much easier to seal up as well as build. You may find it difficult to get a round firebox big enough and then have it properly mounted with enough clearance to stay off the ground as you move your trailer.
That is a darn big piece of pipe and you will have a lot of work ahead of you with that thickness. Should work great to retain heat but will take forever to heat up so plan on adding 60 min to your cook time. What type of smoker are you building?
Building an Offset Reverse Flow. We have a lot of 34" and 24" pipe schedule 30 and 40 from jobs. They intend to send it to the scrap yard so I thought I'd try and see if I can make a go at fabricating sale-able smokers. I'd also like to see if I can start up a food truck or trailer as well, maybe start with some farmers markets or something and see if it goes.
The firebox seems big enough visually but the calculations say it's only 66% for the size of the cook chamber. I don't see how making the firebox any bigger will make any difference, perhaps you can explain.
I find it difficult to post any pictures and perhaps will try the facebook site. Would that work?
It's all about balance and air flow. A firebox too small requires a hotter fire and you'll hunt to keep a consistent temperature. One that's too big and you won't get good clean combustion and a dirty smoke. Size it right and you'll be sitting at the sweet spot, getting a clean burn and consistent, dependable temps along with smoke quality.