I believe you're referring to double wall metal insulated / Class-A chimney sections. Those will not hold up very long as a heat riser.
Probably the easiest heat riser to make yourself is one stacked up with IFB (insulated fire brick) on edge, for a 2.5" riser wall thickness. Some folks stick the IFB together with clay slip. Mine has the IFB mortared in place, mortar made of a 1:1 mix of fire-clay and sand.
If you can't locate a source of insulating firebrick (they are an expensive specialty item), common "split" firebrick laid up on edge so the wall is 1 1/4" thick and wrapped with rockwool insulation makes a good riser.
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