I am currently in the planning phase to build a replacement furnace. The outer shell is going to be part of a 120lb gear oil drum. The diameter of the drum is approximately 14" and it will have 2" thick walls. Taking the advice I received on here earlier, my current plan is for a 1" hot face of Kast-o-Lite 30 backed by 1" of fiber insulation. While my current plan is to cast the base as a solid 2" thick disc, would it be better to add 1" of ceramic fiber to back the cast base as with the walls or would it not make much of a difference?