how to improve self-constructed calorimeter (heat insulation)

Purpose: My purpose of this project is to use some environmental friendly material to construct calorimeter instead of using styrofoam.

I tried: I use two different size test tube as my calorimeter(as in picture). And I tried two ways to improve heat insulation: 1.vacuum 2.silvered But, these are not good enough to prevent heat loss.

Question: 1.Is there any suggestions to improve heat insulation. 2.Increase the space between two test tube would sufficiently prevent heat loss?

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