All the installations I have seen use a standard vacuum flask with no
additional insulation, but they were all buried either beneath/behind
the instrument panel, or inside the fuselage - anyway in shadow.

A key element I have read about, and all mine have been fitted with, is
copper pan scourers loosely filling the interior of the flask, so that
temperature variation within the capacity is elinimated.

However, I am no expert, and I look forward to some who are telling how
it really should be done.

Chris N.