I have a 250 gallon propane tank that I will be making into a reverse flow in the next couple weeks. I have researched a lot about the insulated fire boxes and figured that is the way to go, but I am unsure on what size it needs to be. I looked at other custom pits the same size and they run from 20 x 20 to 24 x 24. I tried using feldons pit calculations but they say it needs to be bigger than that. Has anyone built something similar? Does the size of the fire box change if one is insulated and one isn't? And what type of insulation has yall used? I'm thinking about the ceramic blanket but don't know if any brand is better than others? Thanks!