Calculate thermal conductance of thick walls of a cuboid, given material conductivity and cube size

Note 1: This is probably as much a maths question as physics. Note 2: I am not sure if my use of the the term conductance is correct.

I want to make a kiln. The walls will be thick. I can come to two figures for the conductivity of a kiln of given dimensions, between which the actual conductivity will lie. For example:

Assume: Kiln inner dimensions: 1m x 1m x 1m Kiln outer dimensions: 2m x 2m x 2m Kiln lining material conductivity: 0.5 W/mK

Lower bound kiln wall conductance = MaterialConductivity * Area / depth = 0.5 * 1*1*1*6 / 0.5 = 6 W/K

Upper bound kiln wall conductance = MaterialConductivity * Area / depth = 0.5 * 2*2*2*6 / 0.5 = 48 W/K

My question: in the context of a thick-walled cuboid with consistent wall thickness, how do I calculate thermal conductance? Or more generically:

1. Can I split this up into calculations for 6x 3d trapezoids?
2. How would thermal conductance of the trapezoid be calculated?

Thanks

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