Hello Nefarious! I appreciate Jim referring me to your question, and I'll help any way I can...but really I'm just an amateur with a few builds under my belt...so I'm certainly not professional lol!
So my experience is, it's not a problem to make the FB larger than the calculators call for. Not sure how much oversized you're talking about. If it was a lot larger than necessary, you might end up with a fuel hog...that burns through more wood than with a smaller FB...but I expect it'd still perform well. For myself, since steel, welding consumables, etc. are expensive, I would always try to keep my dimensions close...just to save on materials costs. It's certainly possible to end up with overkill and over-engineering. I guess what I'm saying is keep it as simple and cost efficient as is possible.
Same principal applies with making the FB insulated. That will require roughly twice the steel (and twice the welding) in your design. Unless you live somewhere with very cold winters and high winds, where an insulated FB might come in handy.
Last question regarding round or square. That's mostly a personal preference thing IMO. Lots of guys make their FBs round because they already have round pipe on hand. Myself, I like square fireboxes - mostly because I find it easier to get the dimensions correct...plus I find the fit-up and welding easier as well. Again, that's mostly a personal preference for me. But either round or square will perform well as long as the calculations are correct.
Hope any of that helps! Again I'm definitely no pro...but I'll help any way I can. Just ask if you have more questions. And there are several members here who are more accomplished fabricators than me. Hopefully they'll chime in and add their advice as well.
Gool luck with the build! And don't forget to post a build thread! We like following along through a smoker build!
Red
