250ga smoker build - insulated firebox plate thicknesses?

[pyrex]

This will be the first insulated box I've done on a smoker (30x24x24) My plan was to use 1/4 plate like I would typically use for the fire chamber itself (inner chamber) and then 1/8" plate for the exterior with insulation between the two. It seems to me using 1/4 plate for the exterior is a waste of materials, but I could be wrong. Also, is 2" of insulation sufficient?

 

[pineywoods]

daveomak would be the guy I would ask these questions.

Personally I would think the 1/8" outside would work but if I was building it for myself I think I'd use the 1/4" so it's a bit thicker and would be thicker when it starts rusting as well