The beginning of forge design is the study of burners. The proper end point of burner design is the equipment they power. Lumped together, these subjects are too unwieldy; at the same time, they can't stand-alone, since their study is as intertwined as the equipment itself.
What kind of forge is best is an arbitrary question, without any adequate answer. But, whatever design you favor, there are common factors which lead to a desired end point--or away from it.
Size: Bigger is never
Kast-O-lite 30 insulating castable refractory poured in a 1/2” thick layer gives a large measure of thermal protection, along with mechanical armoring, incase you are moving crucible tongs in and out of your equipment, and for general shop safety; it is use rated to 3000 F, is alumina based for flux tolerance, contains glass mini spheres for insulation, and is very resistant to thermal shock; it ways 90 lbs. per cubic foot (compared to 146 for regular refractory). Kast-O-lit