Ribbon burners are great for even heating but if your used to using a torch to heat your glass a giberson tip or something like it delivers a nice hot spot. I'm trying to follow your post n-dark but not sure. Doesn't matter if you use brick or castable but you will want to back it with fiber. Your cast shell only needs to be a couple inches thick anything more is counter productive. Kastolite 30plus is a great all around castable. Mizzou makes a great floor but has no insulation value. A heat retention ring is a must. This reduces the size of the opening smaller then the interior. In turn it will hold the heat in when the doors are open.